Introduction to Actual Laser Construction

This chapter deals with some of the general issues involved in constructing a
home-built laser. These include those related to the hardware of the laser
resonator itself including structure, optics, and mirror mounts, as well as
the power supply required to run it.

One area that is often overlooked but which adds significantly to the
professionalism and impressive appearance of home-built lasers - not to mention
user and visitor safety - are the safety labels. See the section:
Laser Safety Labels and Signs for examples
of those for common lasers. You may need to modify them for the particular
laser you decide to construct.

Much more detailed information on each type of home-built laser can be found
in the chapter for that specific laser.

The Laser Assembly and Optics

The stability and strength of the baseplate are probably the single most
important factors in determining how easy it will be to set up and maintain
a laser with an external resonator. Multiple optical components have to
remain aligned to a small fraction of a degree despite changes in temperature
and placement (on another surface like a lumpy tabletop) of the laser
assembly.

Forget about most wood - it is too flexible, absorbs moisture and warps or at
least changes size all too readily. It may be possible to totally seal some
high quality wood or wood-based composite products but it probably isn't worth
the effort.

Start with a solid metal base. Short of something milled from a big heavy
casting or the use of a real optical bench or table or a converted lathe bed,
the best is an extruded aluminum box shape since this is very strong for its
weight and will resist bending and twisting. A C-channel extrusion will be
nearly as good if it is braced at multiple points along its open side - and
this is more accessible for attaching screws and whatever from underneath.
Or, a thin removable cover plate can be screwed to the open side.

Buying a big enough piece of this new - say 4" x 2" x 4 feet, more or less
depending on the size of your laser - will set you back a few bucks but will
save a lot of time in the long run.

Drill and tap holes for mounting the laser tube, mirror mounts, and whatever
else you need. With tapped holes, there is less opportunity to spend your
time fishing for lost screws! Add keying holes for assemblies that may need
to be removed and replaced without changing their position - like the mirror
mounts. Attach some non-slip material on the bottom to force the entire
affair to stay put!

The advice has been given to avoid wood as a structural material, but for
experimental use there are times when wood might be the material of choice.
If you need a certain shape that can be made of wood and you don't have a
milling machine handy to make it of steel or brass you might prefer to have
it in wood tonight rather than wait until next Tuesday to have it made of
metal. Threaded holes are easily made in wood. You just drive the screw
into an undersize hole and it makes its own thread! You might have the
objection that wood is not dimensionally stable. Quite so in the case of
bringing in a board that has been out on the woodpile on a rainy day, but
for plywood that has been stored indoors for several weeks in an air
conditioned house there will be very little change. The main need is for
rigidity, and wood can be made rigid. You don't want the mirror to shift
the minute you touch the mirror mount. You need to turn the adjusting screw
on a mirror mount without imparting much translational force. Torque without
Push. What I found helpful was to drill one or two holes transversely through
the screw knob. Then I made a little tool consisting of a 7 inch long 1/4 inch
dowel into the end of which I inserted a straight, half-inch long wire (from
a paper clip). It is used as a capstan wrench to make tiny adjustments of a
screw.

Getting back to wood... If you want something that you expect to keep its
adjustment on the shelf until Christmas then wood is not called for. But
for experimental work, something that's here today and something else
tomorrow, why not use wood?

An example of how simple and crude you can be and still get away with it
was shown in our 1965 paper by Vander Sluis et. al. in Figure 1. (I don't
have the photo but a description of the paper and reference can be found in
the section: K. L. Vander Sluis et. al. HeNe
Laser --- Sam.) It is a photograph of the world's simplest HeNe laser. On
a sheet of half inch plywood there are four chemist's ring-stands in a line.
Mounted on them are four burette clamps. The two on the ends are holding
concave dielectric mirrors. The other two are holding a sealed-off laser tube
about 75 cm long. Lying on the table and serving as power supply is a Cenco
Tesla-type leak tester with a wire leading up to a band on the tube. With
about a half hour of testing and adjusting this contraption was actually
lasing! It may have been going when the photo was made - I don't remember and
the picture doesn't show it.

Another time, just to see if we could do it, a laser was run with no
mechanical support to either mirror. Dr. VanderSluis and I each had an
alignment card in one hand and a mirror in the other at opposite ends of
the laser, both of us trying to hold his mirror in alignment. Every once
in a while we would both be in alignment at the same time and it would
flash. Not a very practical way to go.

There are several considerations when selecting a material to be used for
a Brewster or other window through which light must pass undisturbed:

Optical quality. This refers to the surfaces (plane, flat, and
polish) and purity of the material.

High quality microscope slides (not the kind that are 100 for $1.00 at your
local hobby store) are actually quite good. To check a microscope slide
or real optical flat:

Hold it at arm's length and view a distant scene through it - there
should be no detectable distortion or shift of the image as it is
inserted/removed from the view. Alternatively, insert/remove it from
the path of a laser beam projected onto a far wall (reflecting back from
a mirror to a nearby screen if you don't have a partner). There should
be no noticeable shift in the position of the projected spot with/without
it in the beam.

Viewed edge-on, there should be no coloration (green or otherwise).

When well cleaned and placed in the concentrated beam of a slide
projector or laser, there should be minimal or no detectable scatter.

Index of refraction. This will mostly affect the Brewster angle,
percent reflectance with respect to window angle, and angle of total internal
reflection. The dispersion will also be important if multiple wavelengths
are involved.

Where the angle of a Brewster window is not adjustable (e.g., no bellows
or ball-and-socket joint connection), the index of refraction should be
determined (experimentally or from a reference book or the manufacturer)
so that the it (the Brewster angle) can be set quite precisely (within
+/1 1/2 degree if possible). Assuming an air/glass interface, the Brewster
angle = arctan(n) where n is the index of refraction.

Heat absorption/losses. This can be critical where the window is
part of a low gain laser resonator as in the case of the Brewster window for
the HeNe laser. Quartz should be better than common glass in this regard.

Keep in mind that the light intensity *inside* the resonator is going to be
many many times greater than the actual power in the output beam. This
ratio will be approximately 1/(1-R) where R is the reflectivity of the
Output Coupler (mirror reflectivity specified between 0 and 1).

For example, with a HeNe laser, a typical R is .99. So, the power level
between the mirrors will be roughly 100 times greater than the actual power
in the output beam - or 1 WATT for a 10 mW laser!

Thus, absorption->heat losses can be significant and need to be minimized.
(And no, you cannot stick a mirror in at an angle to extract a high power
beam but think about zig-zag paths through laser gain media if you have
trouble sleeping some night!)

The equation for the Brewster angle defined between the window and a
plane perpendicular to the direction of the light rays (tube axis) is:

Brewster angle = arctan(index of refraction)

For a quartz window - desirable for an HeNe laser due its lower heat losses at
632.8 nm, the index of refraction is 1.54 resulting in a Brewster angle of
57 degrees.

So, this is a piece of cake even if you weren't a stellar performer in high
school trig. However, suppose you don't know the index of refraction of the
material you are using? Ah, no problem if you have a light source (like a
laser) of the SAME wavelength since it can be determined experimentally. For
the construction of the HeNe laser this should be no problem since you likely
already have some sort of HeNe laser! And, we already warned you that you
shouldn't be building the HeNe laser if your goal is just to have a working
HeNe laser anyhow. :-)

The light source has to be polarized. This either means a laser outputting a
polarized beam (by design or see the section: Unrandomizing the Polarization
of a Randomly Polarized HeNe Tube) or the use of a polarizing filter on its
output. However, for the latter, common HeNe tubes produce a beam with random
polarization - it varies as the tube heats up and just because it feels like
it! This means that the intensity will be varying at the output of the
polarizer so this will have to be taken into account as you view the reflected
beam.

Fasten a sample of your window material to a block of something so it is
perfectly vertical and in a way so that it can be adjusted between about 30
and 70 degrees with respect to the direction of the laser beam and that the
angle can be accurately measured.

Line everything up with the window at a 45 degree angle and turn on your
laser.

Rotate the laser or its polarizer about the axis of the beam until the
reflection off the window is minimized (keeping in mind that your actual
beam intensity may be varying at the same time if you have a random
polarized HeNe tube).

Adjust the angle of the window until the reflection is further minimized.

Repeat steps 3 and 4 until the reflection is as small as possible. When
optimal, the reflection should be extremely faint. Measure the angle. :-)

When most people think of mirrors, what they use for shaving or makeup or
the rear or side view mirrors of the automobile come to mind. However, none
of these would be permitted anywhere near a laser lab. To put it bluntly,
their quality and performance stinks!

The ideal mirror would have a coefficient of reflectivity of 1 (100%) for all
wavelengths of interest (no transmission and no absorption), no scatter, and
introduce no (unwanted) distortion. (However, specific reflectivities of less
than 1 over a range of wavelengths are required for laser work as noted below.)

Optical quality mirrors are usually 'front surface' coated which means that
the reflective layer is on the front of the supporting structure rather than
behind as in the case of a common household glass mirror. Otherwise, there
will be a ghost reflection from the front (uncoated) surface and degradation
from passage through the mirror material (glass, plastic, etc.)

For many applications, metal coated mirrors are fine. Aluminum and gold
are common (well, OK, maybe not so common for gold) materials that have
decent reflectivity at a wide range of wavelengths. However, even the best
metal coating has a reflection coefficient of less than 0.99. This may not
matter viewing your face in the morning or even for bouncing a laser beam
around an optical apparatus. But where every last fraction of a percent
counts, another type of mirror coating is needed.

Where performance is critical, the mirrors are not metal coated but are
a type called 'dielectric' (the term 'dichroic' may also be used which is
the same thing). They are made by depositing many alternating layers of hard
but transparent materials having different indexes of refraction. (Since
the coatings are insulators, they are non-conducting - thus the term
'dielectric' as opposed to conducting metal coatings.) The
thickness of each layer is precisely 1/2 the wavelength of the light for
which the mirror is being designed. This results in reflection by
interference with very high (>99.9%) efficiency - much greater than
for even the best metal coated mirrors. On either side of the nominal
wavelength, reflectivity falls off so this type of mirror may not useful
without additional work where a wide range of wavelengths is involved.
By varying the thickness of each layer and the number of layers (some
mirrors will have more than 100 layers!), specific reflectivity and
spectral response can be controlled very precisely.

Mirrors are used in two sorts of places: as part of the laser resonator and
everywhere else. :)

Mirrors external to the laser resonator - used for redirecting the beam,
for example, are a lot less critical since in most cases, the light only
reflects from them once and thus a slight imperfection or fraction of a
percent loss isn't quite as critical (though there certainly are exceptions).

Mirrors which are part of the resonator are special in that they affect the
performance of the laser directly since the light bounces back and forth
between them many times and they affect the gain of the laser resonator.
Percent reflectivity with respect to wavelength, surface figure and finish
(shape as in planar or concave and quality) are all critical.

Dielectric types are almost always used for gas lasers.

Planar or curved mirrors may be used at each end. Where a concave
mirror is used, the focal length (f = 1/2 * r, the radius of curvature) is
usually about about equal to the length of the cavity (L). So, r is
about equal to twice the distance between the mirrors. This is called
a 'confocal' configuration and results in easier (in a relative sort of
way) adjustment of the mirror alignment - it is somewhat forgiving.

Planar mirrors result in higher efficiency in the lasing process since
more of the lasing medium can participate (think of the shape of the
reflected beam inside the tube). However, diffraction losses are higher.
You can't win on all counts! :) A true spherical resonator (L = r) would
be easiest to align but would use even less of the lasing medium.

The use of planar mirrors have a couple of other advantages as well: Putting
a planar mirror at one end allows additional optics to be introduced into
the cavity near that end without requiring much, if any, realignment of
the mirror. A 'folded confocal cavity' with one planar mirror and the
other having r = 2 * L is a good choice in this regard and will also have
the beam waist located at the OC. Planar mirrors are also generally much
less expensive than curved ones (and it may be desirable to experiment with
OCs having different wavelength characteristics and reflectivities so cost
savings here could be important)!

A typical laser resonator will have a High Reflector (HR) mirror which is
very nearly 100 percent reflective (as close as possible depending on its
quality - and cost) and an Output Coupler (OC) mirror which is partially
transparent - it reflects less than 100 percent of the incident light at
the wavelength or range of wavelengths for which the laser is designed.

The OC will typically be designed to transmit anywhere from a fraction of
1 percent to more than 50 percent depending on resonator gain. Some types,
like the copper vapor or nitrogen laser have so much gain that the OC needs
to have little or no reflectance.

In some instances, a laser will use a pair of OCs to get usable beams
(possibly at different wavelengths) from both ends. But note that
there will almost always be some leakage through even the best
dielectric HR unless it is covered with paint or tape or has a metal
back-plate, since its reflectance isn't quite 100 percent. If you have a
HeNe laser and you can see the HR mirror itself, you will likely be able
to detect a weak beam, perhaps 1/1000th of the intensity of the main beam
exiting that end.

For special applications, some lasers use a pair of HRs to maximize the
intra-cavity photon flux (or circulating) power and produce little or no
output beam.

Needless to say, you aren't going to find resonator-qualified mirrors at the
local variety store! Unlike aluminized telescope mirrors which are possible
to coat in your basement (at least in principle), this is not an option for
dicroic types. They can be obtained from optical supply companies and in the
case of the HeNe laser, dead or sacrificial HeNe tubes. (Argon or krypton ion
also, but you aren't likely to have any!)

Note that for testing a resonator, a pair of totally reflecting (HR) mirrors
can be used. This will result in the lowest possible lasing threshold because
very little light escapes through either mirror. Of course, you won't get much
of a beam either! However, as an indication that your laser is working, there
will be some coherent light reflected off of the not quite perfect Brewster
windows and some will leak through the typical HR as long as it isn't covered
with tape or paint or a solid metal back plate as noted above! If you can get
your laser working in this manner, substituting the proper OC mirror that
transmits a small percentage of the incident light should be a piece of cake!

The HeNe, Ar/Kr ion, HeHg, and other similar gas lasers all require adjustable
mirrors outside and usually separate from the tube itself. These should be:

Solid, rigid, and stable with respect to temperature and humidity.

Easy and intuitive to make fine adjustments.

Easy to remove and reinstall with minimal effect on setting (as when doing
initial alignment, for example).

Inexpensive and easy to fabricate. :-)

You can of course purchase such mounts from an optics supplier, but one of
them will probably set you back more than the entire budget for this laser
project! While the homemade alternatives discussed below do work, they WILL
almost certainly be less precise that what is available commercially.
Thus, if you can afford quality mounts or find a good deal, by all means buy
them! (See the additional comments below).

All of the following designs should be adequate for use with home-built lasers.
These mounts consist of a right angle aluminum bracket, an aluminum plate to
which the mirror is attached with glue (around the edge), screws, or clips,
and two or three spring loaded thumb-screw adjusters. Indexing balls between
the base and the mounting surface and an adjustment screw allow it to be
removed and replaced with virtually no change in alignment should this ever be
required. Both designs can be constructed using common hand tools though a
drill-press would be nice and high quality drill bits and taps are a must!

While the drawings show the mirrors themselves glued to the mounts, a better
approach is to construct something to hold the optic that can be easily
removed and replaced without risk of damage. See the section:
Mounting Laser Mirrors for one such
design that constructed easily without the services of a fully equipped
machine chop.

The dimensions given below are just suggestions. Modify them depending on
your particular needs. Using the smallest height which provides the desired
baseline for the (Y) adjustment and then adding a block underneath the entire
assembly to raise the mirror position to center it within the tube bore will
maximize stiffness.

Note that while all the mirror mounts described below show coil springs, I
have found that these can generally be replaced with 1 or 2 split (lock)
washers (which are what I now use for all my home-built mirror mounts).
While the adjustment range is reduced to a few mR, this is still quite
adequate for most laser resonators as long as care is taken in fabricating
the mirror mounts and pre-aligning them on the baseplate and mounting the
optic squarely on its plate or in its holder. However, if your machining
skills are somewhat rusty, go for the springs. :)

The Adjustable Mirror Mount 1 is similar to the
one in "Light and its Uses" and uses three thumbscrews equally spaced around
the mount (at the vertices of an equilateral triangle). The disadvantage of
this approach is that operation isn't terribly intuitive since the X and Y
axes are not independent. However, the screw at the top does pretty much
only affect Y. Then, all you need to do is remember to always turn the two
other screws an equal amount opposite directions - which will mostly affect X.

The Adjustable Mirror Mount 2 is very similar
as far as construction is concerned but moves the thumbscrew locations to the
corners of an isosceles right triangle. And, although three thumbscrews are
shown, the center one (marked P) can be left alone or replaced with normal
screw, a point contact, ball joint, or something similar, since all
adjustments are made with the thumbscrews marked X and Y. With this
arrangement, the side and top thumbscrews produce nearly independent changes
to mirror orientation in the X and Y axes respectively.

Another easy-to-build design that is very similar to the X-Y kinematic mounts
sold by optics companies is only slightly more complicated:

The Adjustable Mirror Mount 3 separates the
adjustment points and the spring retainers and if carefully made with fine
pitch thumb screws, should be quite precise. The
Adjustable Mirror Mount 4 is virtually identical
but locates the movable mirror plate closer to the laser tube or rod making
it easier to add a boot or bellows to keep out dust! (This modification
could, of course, be done for Mirror Mounts 1 and 2 as well.)

The adjustment and pivot screws in commercial mirror mounts of this type may
have a steel ball glued into a recess at their end to form a highly stable
symmetric tip. If you have access to a lathe, you can do this as well. Or,
just mount an ordianry blunt-end screw tip-out (I'll let you figure out how to
do this!) in an electric drill or drill press and use a file followed by fine
sandpaper or emery cloth to form it into a smooth blunt conical shape.

The conical tips of the thumbscrews and pivot screw press against matching
depressions in the fixed plate. To align everything, first drill holes for
the three adjustment bushings (slightly undersize if neceesary for a press fit)
and secure them in place (press fit or threaded nut as appropriate, or glue as
a last resort). Then, drill one of the holes for the spring retainers. Use a
snug fitting nut and bolt to clamp the two pieces of metal together and then
drill the other retainer hole and add a nut and bolt there. Make sure they
are tight!. Now, using the holes in the threaded bushings as guides, drill
the depressions for the tips of the thumbscrew but make sure you only go about
halfway through the fixed plate - set your drill stop to this depth. Also
take care to avoid damaging the threads on the bushings in the process.

For the assembly to be stable, all three screws (X, Y, and P) must seat in
the bottoms of their matching depressions. If the slight 'give' between the
screws and bushings isn't enough to assure this, it will be necessary to
elongate the depression at X ONLY in a direction parallel with a line passing
through X and P and widen the one at Y in the appropriate direction to allow
the screw at Y to seat properly (X and P will fix the position of the plate;
the depression at Y can be widened in all directions or left out entirely).

The Adjustable Mirror Mount 5 (which I haven't drawn but have built)
consists of a rigid baseplate made from right-angle aluminum stock and a pair
of hinged flat plates - one on top of the other - attached to it. The X
adjustment (first) plate is hinged on the side and the Y adjustment plate is
hinged on the bottom. The hinges can be actual leaf-type hinges but a pair of
point contacts will work as well or better. For these, I just drilled a pair
of holes suitable for a 6-32 tap through the hinged plate and to a depth half
way through the mating plate and threaded a short 6-32 screw through the hinged
plate. When the tip of the screw is forced into the slightly undersize hole,
it forms a very stable pivot for the couple degrees of freedom that is
required. A thumbscrew for each axis is threaded through the hinged plate for
that axis and bears on the plate behind it. Each plate can have its own spring
(and optional locking screw) to maintain it in position or a single
strategically located spring can hold the entire affair together (I tried both
approaches but prefer the former).

For any adjustable mount, the tighter the pitch (threads/unit) of the adjusting
screw and the longer the baseline (distance between the screw and the relevant
pivot in units), the finer the control of mirror angle. Units can be inches,
mm, etc. The sensitivity of the adjustments measured in mR/turn is given by:
S = 1/(Baseline * Pitch).

Of course, more precise machining all around will reduce the amount of wobble
and other random unpredictable behavior. In addition to the sensitivity of
the control, repeatability is an essential characteristic of a laser mirror
mount.

In addition to the thread pitch (see below), the length of the bushing, the
quality of the match of its threads with those of the thumbscrew, as well as
the size of the adjusting knob or length of the adjusting wrench will determine
the precision of these mounts. Up to a point, a longer bushing and larger
knob is better but almost anything beats a simple nut and tiny headed screw!

The addition of a locknut or setscrew, and/or removal of the knob (without
disturbing anything), will reduce the possibility of adjustments changing on
their own.

While the ALC-60X has rather mediocre per-turn sensitivity, its adjusters are
tight 5/8 inch nuts requiring a wrench for adjustment so they actually end up
being much better than might appear based simply on thread pitch and the size
of the baselines.

For higher quality components than available at the corner hardware store, go
to Thorlabs and search for 'taps' (or
get a Thorlabs catalog). They have the 80 pitch screws, taps, and other tools
and parts that you need to make your own more precise mounts. Prices aren't
that terrible either considering what you get. For example, 1/4-80 thumbscrews
and nuts (actually tapped bushings) are $6 to $9 and $6 to $7.90 respectively
(depending on length in each case). The 1/4-80 tap is $12.60 if you want to
make your own threads. Then, the incremental cost of an adjustment will be
only $6 (assuming the 1 inch thumbscrew - which should be adequate for these
lasers). However, the bushings may be less hassle since getting these fine
threads to mate smoothly over any length may be difficult. Other possible
sources this sort of hardware may include
Melles Griot and
New Focus.

(From: Steve Roberts.)

Buy an MM1 from Newport or a KM1 from
Thorlabs and then see if you really
want to try to clone it. There is a reason for the traditional kinematic
design of the mirror holder. The ball shaped pivot, the cone and the flat,
and the specially milled groove, are there to eliminate crosstalk between
the X and Y axis. Buy one, look at it and you'll see what I'm talking about.
Then you'll see why they get $40 to 80 each for the 2-3/8ths thick one inch
square blocks of aluminum. If you have a milling machine or access to one,
or don't mind crosstalk, then making your own fine mounts is child's play,
otherwise, for long distance applications, you'll find yourself willing to
pay for the quality units once you've used them. If you need really large
mounts, then making them yourself becomes a viable option.

The most critical optical components will be the HR and OC mirrors which are
part of the laser resonator. While some lasers like the CuCl/CuBr and N2
lasers only require simple mirrors (e.g., microscope slides) or no mirrors at
all, for most of the others, the specifications of the mirrors (reflectivity
versus wavelength, curvature) and quality will to a large extent determine the
potential performance of your laser - and even whether it will lase at all!
This is an area where you cannot cut corners. Except for the CO2 laser where
it may be possible to grind and coat your own mirrors, this means buying them.
There are a variety of possible sources for these mirrors:

Laser and optics companies. Needless to say, manufacturers of laser
systems and components (not just integrators who pop a HeNe tube into a
scanner), know the most about high quality mirrors. Anything custom is
going to be *very* expensive (figure $1,000s) but the prices of standard
mirrors may not be that bad. For example, an optics set for an ALC-60X
from American Laser Corporation may be under $200. If you are persistent,
get in touch with the right person (a technical type, not a bean counter),
you may even be able to get something at a steep discount or free. Be honest
about what you are doing - you represent a future paying customer. But, you
have to convince them you are serious and know what you are talking about,
have an idea of what you want but be prepared to compromise. You may have to
alter your design to accommodate an optic that might be available at a good
price.

Laser surplus outfits. Sometimes, suitable optics will appear from
suppliers of lasers and other types of laser components. However, this isn't
that common.

Salvage. There are a variety of types of equipment that may contain
suitable mirrors or optics, and in some cases, even structural components
and adjustable mirror mounts. Of course, for equipment like laser printers,
there will also usually be a complete laser of some sort buried in there
somewhere (or was until someone else got there first)!

HeNe and Ar/Kr ion laser tubes or laser heads. That dead HeNe tube
likely has a good set of mirrors. At least one of them might be usable in
your home-built laser. Ditto for internal mirror ion laser tubes and, of
course, external mirror laser heads (though you are a lot less likely to
stumble upon one of them).

However, one or both mirrors may not be planar. A curved mirror can be used
in a *shorter* laser but not in one that is much longer than where it came
from. In addition, the mirror reflectivities will have been optimized for
the particular tube length, gas fill, and configuration (internal mirrors
or external mirror(s) with Brewster windows - and may not be adequate for
an external mirror resonator. Of course, if you found a laser of the same
type as you inteded to build that was dead because of a leaky tube, there
may be nearly no remaining challenges!

Laser printers and similar equipment. This sort of equipment often has a
variety of useful mirrors, lenses, and other optics.

There *are* nice first surface mirrors in laser printers. They are going
to be coated for the IR laser diodes used (around 800 nm unless you have a
really old one using an HeNe laser). Or, if you happen on a high
performance graphics arts copier/whatever using an argon ion laser, the
mirrors will be optimized for that blue/green wavelengths (but you did
remove the laser and its power supply as well, right?).

The planar dielectric mirrors found in an older HeNe laser based laser
printer may be of very high quality and suitable for one of the mirrors of
a HeNe or krypton ion laser. However, since these are generally planar,
using them for both the HR and OC would make alignment more difficult.
Even the aluminized mirrors might be useful in a pinch - I've gotten a
commercial one-Brewster HeNe laser head to lase using one of these. They
would certainly be fine for several of the higher gain home-built lasers.

The ones I've ripped out of IR laser diode based laser printers do appear
to be decently reflective at all visible wavelengths though they do have a
slight orange tint in reflection. They are excellent at the 632.8 nm
wavelength of a HeNe laser. Most of the printers I have seen appear to use
metal coated mirrors - not dielectric. So they won't be as good as proper
dielectric types and are probably unsuitable for use inside a laser resonator
unless it has a very high gain. I've seen dielectric type mirrors in older
HeNe based printers. But even there, the polygonal scanner mirror was the
metal coated type.

Note that some of the fixed mirrors may NOT be planar though they might
appear to be so at first - even that long narrow mirror next to the output
aperture may have a slight curvature in the cross-wise direction.

CD, DVD, LD, MD, and other optical disc/k players and drives. These have
a variety of small optics including first surface mirrors, beamsplitters,
and short focal length lenses.

However, the mirrors at least tend to be dielectric coated for the
particular wavelength being used - 780 nm for CDs, 650 nm for DVDs, etc.
(Mirrors for the 780 nm wavelength in particular usually appear nearly
transparent to visible light.) Again, these mirrors are not likely
suitable for use inside the resonator but fine for redirecting the beam.

Barcode and other laser scanners. Optics include mostly mirrors and
lenses, but there could be some other strange stuff. A combination of metal
coated and dielectric mirrors may be used depending on wavelength (both IR
and visible are common) and the whims of the designer. Quality varies
depending on whether it is a $50 barcode scanner or a $50,000 high
resolution film recorder. :) Other comments are similar to those for
laser printers, above.

Here are some possibilities for laser quality specific wavelength or
broad-band mirrors:

Research Electro Optics (REO) in Colorodo. They make most of the optics
for air-cooled lasers and have very nice white-light sets. (Note that
white-light optics usually kill the 530 nm lines of krypton so you might want
to look at the optics curve before you buy.) These guys are the ones who make
the super polished substrates. However, REO won't sell to you unless you
know exactly the transmission you want.

Balzers (now part of Oerlikon)
makes them also, but
getting a single set from their US office proved to be impossible. I had to
the email their home office in Europe just to get a reply.

I've had good luck using some flat optical stock sold by
Optical Coating Laboratory (OCLI) (now
part of JDS Uniphase) in a pinch for HRs. It's 0.5% transmission or less
across 400 to 700 nm.

Another potential source is high-tech surplus stores: I've gotten some really
good deals on Newport mounts that were built into equipment that was being
scrapped. I've also picked up some mounts that were custom-made for the
equipment, but can be adapted for other purposes. And all but 1 or 2 of the
support rods on my optics bench started life as shafts or ways in machinery.
Most of 'em were even already tapped for 1/4-20 screws on the ends and cost
$1 or $2 instead of $10.

The best deals are usually found when you're at the store and they have some
subassembly that they're vacillating about tearing down into components, but
one surplus place I know of with an on-line presence that sometimes has some
optical bench bits are:

And, finally, there's me. ;-) Maybe. I have some extra odds-n-ends that I
probably ought to sell off. If you drop me an email with more details about
what you need, I'll see if I have anything excess that matches.

These procedures are a means of using the optic (mirror) to image
a source at infinity thus providing the focal length and, from this, the
radius since for a mirror, r=2*f. With care, an accuracy of better than
+/-5 percent can be achieved and probably much more.

The most convenient light source to use is a HeNe laser which has a 5X to
10X beam expander telescope mounted on its output. Some inexpensive
"educational" lasers come with one and they can often be purchased as an
option. Someone gave me such a laser that was being thrown out!
Or with care, a small spotting or finder scope or monocular (half a
binocular!) can be mounted in front of a HeNe laser (eyepiece first) to
expand the beam. The expander should be adjusted for as parallel a beam
as possible. This is one good hobbyist use for an HP 5501 or HP 5517
laser head as they have superb beam expanding/collimating optics!
Other types of lasers can of course be used, even a laser
pointer. If a laser and/or beam expander isn't available, any bright
light source will also work if it is at least 10 to 20 times the distance
of the expected focal length of the mirror since this will be close enough
to parallel to get an accurate measurement. The Sun is also suitable
and is safe to use for small (e.g., 6 mm diameter) mirrors but realize that
the focused reflected light from a large mirror will tend to set things
on fire!

Orient the mirror to be tested slightly off axis at the laser or other light
source so that the reflected beam is easily accessible. Place a white card
in the reflected beam and move it along the beam axis locating the position of
smallest spot size or sharpest focus. The distance from this point to the
mirror its focal length, f. If the beam expands faster than without the
mirror, the optic is convex (convex laser mirrors aren't that
common but see below) and the location where the size doubles will
be -f. In either case, the RoC is 2*f. If the beam size/divergence
stays about the same (compared to the unreflected beam), the optic is planar.

Here's basically the same procedure in different words:

(From: Steve Roberts.)

Take a working HeNe laser, upcollimate it to at least 10X the size of its
normal beam, and make sure it has 1/10 the normal divergence, in other words
just expanding it with a lens wont work, it must have all the rays in it
parallel. Or, take a large diameter source of projected light focused at
infinity, and aim it at a slight angle from the normal to the optic. (Normal
means at exactly right angles to the surface.) The optic should be many feet
away from the light source. Then you should have the beam coming back toward
the source but not hitting it. If it's a flat or convex mirror, the beam
will continue to expand. But, if it was figured with a concave radius during
polishing, by sweeping a card through the reflected beam you can sometimes
find a focal point. Measure the distance from the focal point to the surface
of the optic, this is 1/2 of the radius so double the measurement to get the
radius. This isn't that accurate, but it will give a measurement within 10
percent. You are probably never going to find a convex ion or HeNe optic, but
you might find them in CO2 or YAG lasers.

For ion laser optics, standard radii are flat, then 60, 100, 200, 300, 400,
800 cm. Generally, for a TEM00 beam, the focal length of the output mirror
is at least twice the length of the plasma tube if the rear mirror is flat.

The following is from someone who is involved in commercial laser repair
and even he has problems finding suitable low cost replacement optics!

I recently went hunting for laser optics. A pair of standard coated 12.5 mm
diameter mirrors for an ion or HeNe laser would set you back $1,200-2,000 a
set, you might get a suitable rear mirror for a Hg, CuBr, or CO2 for much
less, but the price of optics for any gas laser will be prohibitive. Large
frame argon laser optics, if you could find a used set, are going to be $250
for optics if the coatings are still anywhere near useful, and much more if
they are in good shape. Costs seem to stay the same regardless of the
substrate material or diameter - buying a smaller optic won't be that much
less expensive if at all.

If you are thinking about going direct to a supplier of laser optics, off the
shelf optics similar to what they coat for other laser companies are generally
not available as the contract prohibits the optics company from selling them.
Thus yours will be a 1 off custom run. The low cost Chinese optics companies
do not do ion or HeNe coatings with the needed levels of reflectivity or
quality. I tried that too, and I have especially good relations with one of
them.

You have to rip them out of a dead laser of similar size and power, and for
HeNe this is a problem as modern sealed HeNe tubes may use at least one mirror
that is concave and is only good at the same working distance between the
mirrors used in that given tube.

I know, I just spent two months hunting down an ion set, $750 an optic new, so
$1,500 for a full cavity for a 1 meter-class laser. That was a relatively
inexpensive optics set too. It was for krypton - I could have bought a whole
used 1.7 W argon ion laser for not much more.

Using the short radius semi-confocal cavity optics of an ALC-60X or Omni-532
(their radius of curvature is around 60 cm) for the Scientific American tube
will not work even though the mirrors are only $300 a set for cheapies.
Mirrors are coated to a specific transmission based on tube length, a small
air-cooled might be .6 percent transmission, where a 2 meter long large frame
25 to 30 watt would be around 8%, but that percentage would be tailered across
the range of lasing wavelengths for a specific balance. So if you tried using
a 2 watt pair of optics for a 10 mW homemade laser, you would be very sadly
disappointed in the output and/or it probably won't lase at all, or if it did
you would only see the ultra high gain 488 line lasing. However mirrors for a
shorter low power laser might work if you scale up the tube. The problem will
be the radius of the mirrors, not the transmission. For example, a 1 meter
radius ALC-60X OC might work for Scientific American ion laser, but the usual
standard 60 cm radius would not. Plus aligning a non optimized cavity would
be a bear, and with a low gain amateur tube, highly unlikely. Funny how the
author left the optics specs out entirely!!!!

For a recent project we put two 45 cm radius optics from a laser with a 1.5
inch longer resonator then a 60X into a 60X, alignment time approached 1 hour
instead of the usual 5 minutes, and did not get any quicker. There was
exactly one path with respect to the bore that worked, including the offsets
in length caused by the X-Y adjustment screws on the end plates, talk about
critical!! Only reason we did it was we needed gain on a line not supported by
the 60X optics for a experiment.

So what I'm trying to say, is, unless you have the right optics, you are
better off investing in a working laser if you are trying HeNe, or Ar or Kr
ion.

There are only 3 companies in the US who produce hene mirrors, and the one
of them that was hobbyist friendly just told me, "no more" as they are
tired of coating optics that get returned with the claim "well my tube is
good, so it must be your mirrors that aren't working, or for argon, I don't
like the green-blue-red balance or transmission of these optics."

(Portions from: Anthony Paolini (apaolini@cros.net).)

I have 2 sets of mirrors to be used for my home-built argon ion laser.

One is a newer hard-coated set from a commercial large-frame argon ion laser I
got from MWK Laser Products spec'd as
100 cm radius coated for 450 nm to 530 nm reflectivity. It is an HR/OC pair
and is in excellent condition but actual reflectivity is unknown. The
reflectivity of the OC is probably under 95 percent and way too low for the
SciAm ion laser. Testing would require a laser with a wavelength in this
range (preferably at 488 nm or 514.5 nm) and a laser power meter of some sort.
Almost any would do as long as relative readings could be taken of the
laser beam before and after it passes through the mirror. Then, percent
reflectance is equal to:

The second set is a soft-coated flat/120 cm centered at 488 nm from North
Country Scientific. These were in fact manufactured specifically for the
SciAm argon ion laser. North Country is going out of business and just
selling its remaining stock. (They may have mirrors for the other SciAm
lasers as well.) The mirrors I have would likely have been fine
20 years ago when they were manufactured. But, with the naked eye, pits and
scratches are obvious. I am convinced they are useless. However, other
samples might be in better condition since with proper storage, they can
survive for a long time.

As far as new ones, Esco Products
has "off the shelf" laser mirrors, angle 0, coated for 488/514 nm. (They also
have 633 nm (HeNe red) and 532 nm (doubled YAG green) mirrors. These
mirrors available from stock for $58.00 (January, 2000) which is not bad, but
rear surface is described as "fine ground" making them unsuitable for the
OC even if the reflectance is acceptable as well as complicating alignment.
Also note that they are all planar. Custom mirrors are available but of
course likely at much greater cost.

ALL of the mirrors used inside a laser cavity are extremely
delicate and easily damaged by contact with their surface or improper or
excessive cleaning. Therefore, proper precautions must be taken to avoid
costly mistakes:

The fewer times mirrors are cleaned, the longer they will last.

Avoid touching the coated mirror surface or allowing it to contact
anything. If a fingerprint or anything else gets on it, proper cleaning
will be required. If the other surface is anti-reflection (AR) coated,
it, too, is should be treated with respect but AR coatings are orders
of magnitude more robust than mirror coatings - and don't usually end
up inside the laser cavity so damage to them doesn't affect performance
nearly as much.

Avoid the temptation to wipe off mirrors with *anything* except as
part of a proper cleaning procedure. Use low pressure *clean oil-free*
compressed air or a photographic air-bulb to blow off light dust. Even
the use of a camel's hair brush is generally a bad idea as it may
transfer contaminants to the mirror in addition to scratching the coating.

If not labeled, at least put an identifying number on the ground edge with
a pencil and make an entry in your log book with the complete specifications.
Many of these mirrors look alike but have very different radii of curvature
and reflectivities.

When not installed in the laser or other equipment, store the mirrors
in their original packinging until ready to use. If the mirrors were
salvaged from other equipment, store them in a sealed plastic bag or pill
bottle in such a way that contact with the coated surface is avoided. One
way to do this is to wrap the mirror in a tight fitting paper tube.
Include a bag of fresh dessicant to absorb moisture. (Make it a
habit of saving all those little bags of dessicant that come with computer
boards and disk drives in a sealed container!) If a mirror
must be parked temporarily, place it non-mirror-side down on a new sheet
of lens tissue.

Use a dummy piece of glass or an old damaged optic for initial setup of
your laser or whatever to prevent accidental damage to the good stuff.

Fabricate a suitable holder for the mirror glass itself that minimizes
the risk of finger contact (e.g., the coated surface is recessed) - or
dropping! Leave it in the holder at all times if possible. See the
section: Mounting Laser Mirrors.

If small mirrors are to be used in standard lab mounts, simple adapter
rings should be purchased or fabricated.

I've made adapters to allow 7 to 8 mm diameter mirrors intended for,
or salvaged from HeNe laser tubes to be used with standard 1/2" Newport
mirror mounts (e.g., U50-A). A piece of 1/2" aluminum threaded spacer
(or rod stock) was sliced into 1/2" sections with a hacksaw and miter box.
After smoothing, a 5/16" (7.93 mm) hole was drilled through the center of
each section. A 2-56 tapped hole was added on the side for a Nylon
set screw. These also allow for easier handling of the small easily
damaged mirrors.

NEVER use solvent-based adhesives anywhere near optics. The solvent
may leave a film on optics surfaces. Above all, NEVER even think about
using any cyanacrylic (SuperGlue)!!! Three tiny dabs of 5-minute Epoxy
around the perimeter can be used to safely attach mirrors and other
optics to mounting plates.

Some sort of boot or bellows arrangement is desirable in the laser
resonator to minimize the possibility of dust and other contaminants reaching
the mirrors. However, mirrors mounted vertically or with the coated surface
facing down will remain quite free of dust at least in the short run even
without a sealed cover in a reasonably clean environment.

Where cleaning is required, follow proper procedures and avoid cleaning
unless absolutely necessary - each time optics are cleaned, there will be
some degradation! See the sections starting with:
Cleaning of Laser Optics.

The A HREF="http://www.almazoptics.com/">ALMAZ Optics, Inc. Web site has
summaries of technical data for a wide variety of crystalline and amorphous
materials used for optical components like lenses, prisms, mirrors, windows;
and for optical processing such as polarization and frequency multiplication.

Note: This is only permitted once all resuscitation options have been
exhausted and the appropriate chants and incantations to your "gods of dead
lasers" have been issued. Otherwise, bad things may happen to the Universe. :)

The most likely parts useful for home-built lasers or laser experiments are
likely to be the mirrors, capillary, and possible other glass work.

Either the mirror itself or some portion of the mirror mount assembly can be
removed from the tube. It is generally better to keep the mount intact unless
you intend to build a new mount for it (see below). This is more convenient
for attaching to your laser and minimizes the possibility of contamination
or damage to the delicate inner surface of the mirror. However, cleaning
of the mirror inside the mount using any of the approved laser mirror cleaning
techniques discussed elsewhere in this document is virtually impossible so
should cleaning be needed, the glass will almost certainly have to be removed.
There is one option that might work though. See the section:
Cleaning Mounted Laser Mirrors.

The mirror (glass) will be attached either with Epoxy (e.g., TorrSeal or
equivalent) or glass frit (a sort of solder for glass). Most modern
tubes use frit seals. (Direct glass-to-metal seals are generally not
used for mirrors.) While both these adhesives are in theory softer than
the glass itself, damage is quite possible if much force is used in
disassembly.

Epoxy can be whittled away with an Xacto knife or single edge razor
blade. Unfortunately, the glue may be stronger than the glass. Since
some of the Epoxy will likely be in between the mirror and mount and
thus be inaccessible, it may not be possible to remove it all with the
blade and the glass may end up failing with big chunks of mirror stuck
to the tube or mount. My success rate has been high where the mirrors
were glued to metal end-caps as with the Spectra-Physics 084-1 HeNe
laser tubes. However, for older tubes with glass-glass Epoxy seals, it
has been very poor.

Another technique that may be successful is careful heating of
the metal part of the mirror mount using a small butane torch with a needle
point flame that avoids the glass directly. The glue should melt
at a lower temperature than the mirror glass or its coating - hopefully!

For frit on tubes with a thin frit line like Aerotech and Uniphase,
careful scraping with a sharp tool (e.g., an awl or small flat blade
screwdriver) will permit the frit to be removed without damaging the
glass. Unlike Epoxy, frit doesn't wick in under the glass to any
significant extent so once the visible frit is gone, the mirror glass
will simply pop off. It takes only a couple of minutes. (In fact,
the ease with which the frit can be removed is a bit disconcerting
considering that gas/vacuum integrity depends on it!) The success
rate will be nearly 100% if care is taken to maintain a grip on the
mirror with one hand while scraping so that when the glass does come
free, it won't fall on the floor or contact the sharp edge of the mount.

This technique may be harder for Melles Griot, Siemens, and other
tubes with a fat frit line. I'm told that squeezing the mirror mount
in a pair of Vice-Grips(tm) (locking pliers) or a vice will cause the
mirror to pop off intact about 50% of the time. The other 50% of the
time it will probably break in half or worse. This may be worth the
risk for those tubes with a fat frit line though.

However, I consider such treatments to be cruel and unusual punishment.
Thus, salvaging the mount intact may be preferred. In any case, take
special care that no damage occurs to the mirror (or what's left of it)
when it finally comes free. It may be best to work with the tube or
head upside-down over a soft cloth so that the mirror will fall away
rather than toward the sharp edges of the mount. If I had $1 for every
mirror I've ruined because the fragile coating came in contact with
something just as it popped off.... Dropping it on a concrete floor
would also be bad news.

On one HeNe tube that used an Epoxy sealed mirror to a glass stem, I scraped
away as much of the Epoxy as I could. But when the mirror was cracked loose,
most of it remained stuck in the Epoxy, which was apparently stronger than
the optical glass. Only a 1 or 2 mm area in the center survived. It does
lase though using my one-Brewster HeNe tube. Next time I salvage a mirror
from one of these tubes, I think I'll leave the Epoxy intact but cut the
glass stem a half inch or so away from the mirror.

The mirror mounts on internal mirror HeNe and argon ion laser tubes can
be removed either at the restricted region by filing a groove to weaken it
to the point where the mount can be snapped off or by retaining a larger
portion of the tube including the metal end caps - requiring that the glass
of the tube be cut. It may be possible to just tap on the metal end-caps of
some smaller barcode scanner type tubes (away from the center - toward the
ends - with a small hammer or other object) and fracture the seal fairly
cleanly without totally shattering the glass.

The main advantage of keeping part of the mount is that it protects the
delicate coated inner surface of the mirror from damage. However, it is
virtually impossible to clean in there should the need arise in the future.
If your tube used Melles Griot style locking collars, these can be reused
and clamped or glued to a plate to securely hold the mirror mounts in your
experimental laser permitting easy installation and removal. I store these
assemblies with a piece of tape over the hole to keep out dirt and dust. The
exposed sticky surface will also tend to capture any dust floating around
inside. (There is some risk of outgassing from the adhesive contaminating
the surface in the long term but I have never detected any problems.)

However, for polarized tubes which have internal Brewster plates behind
one of the mirrors, removing the mirror glass itself may be the only
viable option as it may be impossible to extract the Brewster plate and
it's holder from the tube-end. A very few will just slide out but most
are installed in a larger size region of the mount just before the mirror
glass is put in place. If there is no desire to salvage the Brewster
plate, it might be possible to carefully destroy it so the pieces just
fall out without damaging the mirror but that's risky since they are
usually almost touching the mirror.

WARNING: Do not file, grind, or saw any ceramic parts of an ion laser tube
They may be made of beryllia, a nasty biohazardous material. See the
additional comments on this topic in the chapters on argon and krypton ion
lasers.

In all cases, DO NOT saw, file, or sand anything once the inside is exposed -
use a glass cutter and then crack the tube or chip away at it, at least not
without plugging the hole with a wad of tissue or a ball of cotton (but don't
let anything touch the mirror, it's just there to block contamination). Any
dust would result in tiny particles getting on the mirrors which could cause
damage and be difficult to remove if the mirrors are kept on the mounts.
Filing before the vacuum is breached is fine. As soon as you have access to
the inside of the mirror mount tube, put a piece of masking or electrical tape
over it to prevent contaminants from reaching the mirror. Ideally, no
cleaning of the mirror should be needed if it was in a the tube's sealed
sterile environment.

The capillary of a HeNe tube can usually removed mostly intact using a
triangular to score it at the desired location and then snapping it. Other
glass parts may require more creative techniques to avoid breakage.

Electrodes, filaments, getters, and the like may be salvageable as well.

For home-built HeNe and some Ar/Kr ion lasers, the use of salvaged mirrors
from internal mirror laser tubes is often the cheapest way to obtain
suitable high quality optics. Salvaging the mirror and mount together
minimizes the chance of damage or dirt on the delicate inner mirror surface
but should cleaning be needed, it becomes almost impossible using any of
the approved laser mirror cleaning techniques. The normal solvents like
isopropyl or methyl alcohol, and acetone take too long to evaporate and
there's no way to really get a piece of folded lens tissue inside the
mount so it can wipe the mirror reliably.

The only approach I've found for cleaning mirrors inside mounts
that has any chance of working is to use fast evaporating solvent in a spray
can such as electronic degreaser or tape head cleaner. These will not damage
the mirror coatings and evaporate within a few seconds which minimizes the
chance of picking up any contaminants from the air. Give the mirror a good
squirt so it's obviously drowning in solvent, swirl it around a bit, then
shake out the mirror mount and let it evaporate completely. A visual
inspection with a bright light or laser pointer should show if there is
any serious contamination still remaining. Repeat if necessary, Obviously,
whatever solvent is used must be as pure as possible. Not all common
electronics cleaning chemicals meet this requirement. Probably few actually
do and even may differ from one container to the next since absolute purity
isn't necessary for their intended applications.

Do not attempt this cleaning approach unless you are absolutely sure the
mirror needs cleaning! It could make the problem worse.
And, since there's no easy way to really know that the cleaning has been
fully successful without testing in a laser, unless this is convenient
without cracking the vacuum or requiring extensive realignment, it still
may be best to remove the mirror from the mount and clean it properly.

The solvents I've tried so far that appear to work reasonably well are
Chemtronics Freon TF, Chemtronics Electronics Cleaner/Degreaser 2000,
and GC Tape Head Cleaner. I still suspect they are leaving something
behind though so no guarantees! Perhaps there is a special optical
spray cleaner intended for this purpose.

I did do a test using the degreaser on a mirror mounted in a mirror cell
that (1) I could install easily in my one-Brewster rig to test performance
and and compare before and after and (2) I could clean properly if it wasn't
successful. The result was encouraging. Two shots were needed but the
the already fairly clean mirror behaved slightly better after treatment.
So the chemical probably didn't leave any significant residue.

Lenses, mirrors, prisms, and other optical elements are often assembled using
one of a number of clear optical adhesives. These range from balsam cement
(common on older equipment) to modern UV cured materials as well two part
Epoxy. There are times when something like an achromat (a color corrected
lens typically consisting of two elements) needs to be taken apart because the
adhesive has deteriorated resulting in milkiness or other artifacts. Or, more
relevant to our interests, an external lens attached with optical cement to
the mirror glass of a helium-neon laser tube (to modify its divergence) needs
to be removed. While not that common, some barcode scanner HeNe tubes use a
negative lens cemented to a standard model tube to expand the beam (presumably
to simplify the later optics). Another situation relates to the
non-destructive removal of a mirror from a soft-seal HeNe tube.

My arsenal of optical cement removal approaches/solvents include:

Xacto knife - For mirrors or lenses attached around their periphery
where minor damage (scrapes, dings) to the mounting surface can be tolerated,
it may be easiest to whittle away the bead of adhesive as far as possible
using a sharp blade. At this point, there may be so little adhesive remaining
that the optic will pop free with minimal force. Even if it doesn't (don't
push your luck!), much less solvent will be needed to eat through what
remains. Of course, this won't work if the entire contacting surface is
coated with adhesive.

Water - While very unlikely to have any effect, it is easy enough to try!

Alcohol - Rubbing alcohol, medicinal alcohol, or pure isopropyl alcohol
may be enough to dissolve some types of adhesives.

Acetone - Nail polish remover or pure acetone will be effective for some
common optical cements.

Lacquer thinner (Also called Epoxy thinner though it will have next to
no effect on cured Epoxy) - This may work on others that alcohol and acetone
don't touch.

Heat - Some of these adhesives melt at a relatively low temperature such
that a gentle flame (from a pocket lighter, not an oxy-acetylene torch!) may
be enough to loosen the bond.

WARNING: The vapors of solvents like acetone and lacquer thinner are extremely
flammable and poisonous and the liquid eats many common plastics.

CAUTION: Even water may damage soft-coated optics (which thankfully are not
common today). Water will definitely ruin many of the materials used for
carbon dioxide optics though.

Sometimes, simple physical abuse will work as in the case of glass optics
glued to a plastic substrate. However, where glass-to-glass joints are
involved, one of the glass elements is quite likely to fracture if too much
force is used. That glue is tough! Therefore, try the other suggestions,
above, before dusting off the 12 pound hammer. :)

Here are some suggestions specifically for the case of an HeNe tube with
a cemented lens. The acetone worked for me. After failing to loosen the
lens with alcohol and lacquer thinner, I let the end of the tube soak in nail
polish remover for about 10 hours at which point the lens just popped off.
The only damage was some slight mottling of the AR coating around the edge
of the HeNe tube's mirror but this didn't effect the performance in any way.
The lens survived in pristine condition.

(From: Equinox (esoteric@pacifier.com).)

The mirrors are probably glued on with optics glue. We used it a lot at my
last employer. It stays liquid until it is exposed to UV light. Acetone is
what we used to dissolve that stuff. You probably already know that nail
polish remover is diluted acetone. If you want to go the route of using
acetone, go to a hardware store and purchase some stronger stuff. Nasty
fumes. You can get a small can for around $8.00.

But there is a far simpler way to get the lens off. It has worked every
time for me, in fact while writing this I saw a tube on my shelf with a
lens, and I decided to try it again to see if it would work. And yes it
worked again.

Get either a lighter or a small propane torch, or maybe a match? Expose the
lens where the glue is to the flame while rotating the tube (to evenly
distribute the heat) Do this for about 3 - 6 seconds, then just slide the
lens off. (the lens may become black from soot from the lighter or match)
Once you have done this, you can try to soak the lens in acetone to remove
the glue. Also with a soft cloth, use some acetone to wipe the end of the
laser tube off. From the ones that I have done, it always seems that the
glue stays on the lens and not the tube. Make sure the acetone is nowhere
around if you use the flame - heating method. Use it once all sources of
flame have been extinguished.

If you want to go the route of soaking the lens, try heating the Acetone on
a small coffee warmer and cover it as it will evaporate very quickly, and it
is extremely flammable, so not to hot. My experience with solvents is that
they work much better when heated.

You can also try to heat the entire tube in a small oven or toaster oven and
see if that will work. Just don't go from a hot environment to a cool or
cold one or the tube may crack.

The following describes the procedure for removing the actual right angle
prism from the WW-II Tank Periscope Assembly. They are about 6 inches long
in a black Bakelite case with a metal bezel and have been sold by various
surplus places for next to nothing (perhaps $8 which is a really good price
for such a nice optic).

This really isn't directly laser related but II couldn't think of another
more suitable place to put it and perhaps it could be useful in a laser
application! :)

Getting the actual glass prism out intact is pretty easy assuming all you care
about is the prism (not the case):

Remove all the visible screws and the bezel.

Cut around one end about 1/4" deep with a hacksaw, making sure to keep
clear of the prism itself.

Carefully break off the end, gently prying with a flat blade screwdriver
if necessary. That should allow enough compliance with the rest of it to get
the prism out.

Chop/chip off the residual balsum cement or whatever it is.

Use lacquer thinner to remove the paint coating the mirror surface
(assuming you don't want the mirror - on mine, it is degraded somewhat
in any case).

This leaves the (probably) silver coating. I soaked the prism in
photographic film or paper bleach to dissolve the metal of the mirror.
I think it is potassium chromate in dilute sulfuric acid? - it's been
about 20 years since I mixed the stuff. Sorry, I don't have the
formula. Check an antique photography book. It took about a half hour.

The mirrors generally used with laser resonators aren't big thin pieces of
glass like a shaving mirror but cylinders between 1/4" and 1" in diameter
and fairly thick. These are difficult to handle without risk of damage to
the delicate mirror or anti-reflection coatings. Whether salvaged from old
HeNe laser tubes or purchased new, it makes sense to provide a safe mounting
arrangement for these optics where suitable housings aren't already present.
If the mirror is already safely mounted in or on something, there is no need
to read any further in this section.

If the mirrors are to be mounted in commercial adjustable mounts (e.g.,
Newport MM1s) but are too small, machining an adapter should be
straightforward: Cut a disk somewhat thicker than the mirror from
aluminum or plastic cylinder stock of suitable diameter, drill a
hole sized to the mirror just fits, and drill and tap a side hole
for a nylon setscrew. Such adapters can also be purchased at
exorbitant cost.

If you have a Southbend lathe and Bridgeport milling machine and know how
to use them, there should be no problem in machining whatever mounts are
desired to house your new or recycled mirrors.

For the rest of us, here is a simple mirror cell design that can be put
together in about an hour (less if you don't care about aesthetics!) using a
drill press and common hand tools. The drill press isn't even essential but
does simplify things quite a bit. This mirror cell can be easily removed and
replaced without upsetting alignment very much though for curved optics,
a pair of indexing pins would be needed (not shown) since any change in X
or Y position will also affect alignment (but this may be useful for fine
tuning your mirror's 'hot spot').

The design is shown in Simple Mounting Cell for
Salvaged HeNe Laser Tube Mirrors. It basically clamps the optics itself
between a pair of plates. Aluminum is what I use but Plexiglas or some other
rigid material would also be acceptable. A metal or fiber washer glued to the
mounting plate centers the mirror while a cushion on the cover plate provides
some resilience as its screws are tightened (just snug!). These fastening
screws may also allow some mirror adjustment if the bottom plate is aluminum
or Plexiglas (which is fairly soft), or if a compliant washer is placed
between the optic and the bottom plate. But, again, don't push you luck when
tightening the screws!

The dimensions in the parts list below are for the mirrors from the
Spectra-Physics 084-1 barcode scanner HeNe laser tube. The size of the
mirrors from other internal mirror laser tubes and external mirror lasers
may differ. Mirrors purchased new or obtained from other types of lasers
may be larger requiring everything to be scaled up to handle them.

4 - Cover plate screws: 4-40 x 3/8". The ends may need to be shortened
slightly to prevent contact with the mounting surface. (I had to trim
roughly 1 thread's worth off the ends using one of those do-everything
crimping tools.)

1 - Centering washer: ID of 5/16" to match OD of mirror. This washer
can be of any material that can be glued in place. Its purpose is to keep
the mirror centered to avoid damage to its delicate surface during assembly
and disassembly.

1 - Fiber, rubber, or paper cushion glued to cover plate to protect outer
side of mirror and provide compliance when tightening the cover plate screws.

1 - HR or OC mirror from SP-084-1 HeNe laser tube. All traces of the
original adhesive should be removed. This is particularly critical around
the outside area of the coated surface which sometimes has some excess glue
which would prevent the mirror from seating flat on the mounting plate
surface.

In keeping with my "never buy anything unless absolutely necessary" philosophy,
I used a VME Bus card cage cover plate for the aluminum stock and hardware
from various obsolete hard drives for the screws and washer. The resilient
cushion was a 3 ring binder paper reinforcement (trimmed to fit). :)

Don't be tempted to use a flat bottom reamer to machine a shoulder in place
of the centering washer unless you are using that lathe or a very
well aligned drill-press (but see the next paragraph) because the mirror will
likely end up being tilted slightly when clamped in place. Using the mounting
plate's surface guarantees that this won't happen. Drilling the proper size
hole in an existing washer (if needed) and gluing it in place is no big deal.
:) And, don't chamfer the edges of the center holes in contact with the
mirror - it must seat on the flat surface of the mounting plate and be held in
place (via the resilient cushion) by the flat surface of the cover plate.

As noted above, the mirror must be free of any glue or frit that could prevent
proper seating. The adhesive on soft-seal mirrors can generally be removed
with a sharp Xacto knife or similar blade taking great care not to damage
the coating(s). However, with hard-seal mirrors, this may not be possible
since bits of frit may remain firmly attached to the glass and are essentially
part of the glass. A slight modification to the design that will work with
either type - and actually provide some additional adjustments would be to
add a flexible rubber cushion under the mirror and only a protective
cushion (like one of those paper reinforcements) between the cover plate and
mirror. Then, the 4 cover plate screws can be used for coarse mirror
alignment. The outer surface will seat square on the cover plate and the
entire mirror can then be moved on the rubber cushion. In fact, I've found
that this scheme using 4-40 screws which form a square only 1/2" on a side is
sufficient for fine alignment of a 12 inch resonator using a one
Brewster Melles Griot 05-LHB-570 HeNe tube! This approach could also be used
with the machined shoulder instead of a centering washer since the coarse
adjustments can be used to compensate for an imperfectly aligned reamer. The
only disadvantages of the added flexibility (in more ways than one!) are that
more things can change over time - and, of course, that you will be tempted to
constantly tweak everything to perfection! Where fine alignment is performed
elsewhere, once the mirror is roughly aligned, put a dab of Locktite(tm) or
nail polish on each of the screw heads to secure it.

For compatibility with all of my home-built mirror mounts, the distance between
the mounting holes is exactly 1 inch. This allows mirrors up to about 3/4"
to be accomodated (with suitable adjustments in washer size and cover plate
screw locations). (With the benefit of 20/20 hindsight, 1-1/8" for all the
mirror mounts would have been a better choice as it would permit the end-caps
of a typical small barcode scanner HeNe tube or those from the SP-184-1 to be
mounted using a pair of screws without the trimming required to fit 1 inch
diameter objects between screws on 1 inch centers.)

I have now mounted 4 SP-084-1 OCs, 2 SP-084-1 HRs, the OC from a 20 mW Aerotech
HeNe laser tube, and the OC from an AO-3100 external mirror HeNe laser in this
manner. The Aerotech mirror was a frit seal type with a bumpy bottom so I
used the rubber cushion under the mirror approach for that one.

The only disadvantage of this design is that the surface of the mirror is
recessed and difficult to clean in-place - but real mirror cells often have
this same characteristic. Perhaps, it will discourage unnecessary optics
cleaning - all cleaning, no matter how carefully done, degrades the surface.
Of course, the plates can be made from thinner material and/or the holes can
be beveled to improve access.

Sometimes, the best deals will get you a large thin mirror when what is really
needed are a few really small mirrors. So, how to dice them up? The
following also of course applies to cutting other thin glass like microscope
slides.

(From: Laserlover (laserlover@my-deja.com).)

I've been doing stained glass for over 15 years aside from all my other
interests and have cut front surface mirrors for several Newtonian telescopes
I've built and here's my 2 cents worth.

There are several approaches:

Visit or call around for glass suppliers who will cut the mirrors for you.

If you have a diamond blade glass saw, good eyes, and nimble fingers,
you can give it a go yourself (but if you did, you probably wouldn't be
asking the question!).

Otherwise, try the following. You will need a half decent glass cutter
with new blade and some thin general purpose lubricating oil.

Find a hard, flat surface to work on and cover with a few sheets of
newspaper.

Wear cotton or rubber gloves so you don't mess up the mirrors with
skin oils when you handle them (skin oil can etch the mirror coatings
permanently if left too long, bummer!) same practice for any optics or
fiber optics.

Cover both sides of mirror with a clear plastic wrap like "Saran Wrap"
to protect the mirror surfaces

Lay mirror flat after making sure no small particles are underneath
which might screw up you cut and waste a lot of expensive glass.

Dip the cutting wheel in the lubricating oil

Score the piece where you want it to break (the cutting wheel will
break through the surface of the Saran wrap).

Using either Running or Ring Star pliers, apply even pressure on both
sides of the score and hopefully the glass will break where you want it.
If you don't have either type of tool, improvise with some MacGyverisms
to apply support on the pivot point (center of the score line) and
even pressure downward on both sides of the score. VOILA !....Done, whew! :)

Wash the mirrors in really hot water and mild solvent "alcohol" baths
or methanol to remove the contaminants.

Better practice a lot on glass microscope slides before tackling the expensive
mirrors.

Personally, I prefer using a reversible process like a mechanical mount as
described in the previous sections. :) However, if you insist on gluing your
mirrors, here are some comments:

Some adhesives are extremely strong and also shrink ever so slightly while
curing. The result may be that bits of glass can actually be ripped from
your valuable optic. Even if this doesn't happen, the position and
orientation you so carefully set up may change on its own. I've heard horror
stories about SuperGlue(tm) doing both of these. The solvent in SuperGlue
can also end up depositing a film on all nearby optics. So I'd avoid it
like the plague. For that matter, has anyone ever found a truly justifiable
use for this stuff? :) The formulation of many optical adhesives have been
designed to address these issues.

RTV Silicone adhesive - clear, white (e.g., "bathtub caulk"), or black, your
choice (though clear probably looks best) is good for optics that may need
to be removed and don't require position stability to a fraction of a
wavelength of light (since it is quite flexible). Again, 3 dabs around the
periphery, not over the entire optic.

Double-sided adhesive ("sticky") tape is another option. In fact, it has
nearly all of the qualities one would want - excellent holding strength,
minimal thickness, no curing time, possibility of removal (or correction of
screwups). However, not all sticky tape is created equal. The high
strength types from 3M are recommended. Some companies use this approach
to mount all their optics (and as you may have discovered, nearly everything
else!).

When I do use adhesive to mount optics, it's usually 5 minute Epoxy. It's
relatively rigid but not rock hard like most long time curing Epoxies.
The work time is short enough to even allow for manually holding the
optic in place until it will stay on its own. And, when used in modest
size dabs, can be removed relatively easily.

UV-cure adhesives are another option - often used by the "BIG BOYS" to
mount all sorts of optical and mechanical parts. These require exposure
to long wave UV and will then begin to set up in a few seconds to a couple
minutes, though complete curing may take much longer. There are a
semi-infinite variety of UV-cure adhesives, each supposedly optimized for
a particular characteristic like adhesion to metals or plastics, rigid or
resilient, transparent or opaque, etc. Normally, a special very expensive
curing lamp is required to perform the magic. The most popular one retails
for about $1,000 from places like Thorlabs and Edmunds Optics but is just
a halogen lamp with a filter to block most visible light and light guide
to direct the UV at the area to be cured. It's also essentially the same
rig used by your dentist to cure the bonding material and many other
adhesives he/she uses on your teeth. But they pay something like 3 or 4
times that exorbitant price from their dental supply house! It should be
possible to build your own for much much less.

(From: Elliot Burke (elliot@hitide.com).)

Designing an adhesive mount for optics is nontrivial. The biggest problem
is the differential expansion between the optic and its mount. If there is
no compliance in either optic or mount, the difference in thermal expansion
can cause stresses large enough to shear the adhesive, if the adhesive is on
the back of the optic. If the adhesive is on the edge of the optic, the
resulting stresses can warp the optic. So, either:

Temperature control the thing.

Make sure everything has the same coefficient of thermal expansion.

Make the mount compliant in the direction of differential thermal
expansion

Design the mount to hold the part without constraining the long direction,
compliantly in short direction.

Design the adhesive bond to have some shear compliance (silicone)

It isn't hard to calculate the stresses due to thermal expansion - this
should be done as a matter of course with all optical mounts. There are
probably other good solutions too. The solutions involving compliance will
also help if the system is dropped or otherwise shocked.

(From: Bob.)

Glue should only be placed on the periphery of an optic, and only in a few
spots. I normally use three dabs, one at noon, one at 4 and one at 7
O'clock, or there abouts. As to what glue to use, in theory, you can
use just about anything. I have seen superglue used with success, but I
would tend to steer clear from such adhesives, particularly on high power
optics, as cyanacrylate has a fairly high vapor pressure, and you don't want
an film that will absorb/scatter light on your optics. I use Norland UV
curable optical adhesive. You can get the stuff from
Edmund Scientific or
"Thor Labs. It's really great for any
sort of optical work, and it's very strong. Sometimes even too strong. After
it is fully set up, I know of no way to remove it. So whatever you glue with
it sure is going to be permanent. The stuff is fairly inexpensive, but
requires UV light to cure. The UV 'cure-ers' that these companies sell are
outrageously priced. I use a normal novelty store black-light. The cure time
is a lot longer, but $3 sure as hell beast $300!

(From: Sam.)

As far as UV lamps, I'm not sure that the long wave UV types for making
minerals glow and so forth are suitable but perhaps those for erasing EPROMs?
Some of the UV curable optical adhesives do dissolve (or at least soften) in
acetone (nail polish remover) or lacquer thinner.

(From: Bob May (bobmay@nethere.com).)

You want to use a flexible glue to attach a mirror to a backing plate. The
reason for this is that a very rigid connection between the two will stress
the mirror to an incorrect shape before the problems of separation due to
overstressing the joint. For larger mirrors, 3 dots of silicone adhesive at
about the 50% radius (the old theory was about 70% of the radius) is about
the right place to put the adhesive. For smaller mirrors, a single dab is
usually sufficient if it's a significant part of the back.

(From: L. Michael Roberts.)

I use standard 'transparent' bathroom silicone from Home Depot. I
have found that 'superglue' becomes brittle over time and 5 minute
Epoxy is almost impossible to remove. A thin layer of silicone
allows thermal expansion and can be removed with a razor blade when
the optic needs changing [although this usually breaks thin
mirrors]. This is not an expert opinion - your needs may differ
depending on the nature of the application.

(From: Louis Boyd (boyd@apt0.sao.arizona.edu).)

Silicon RTV adhesive is reasonable for many uses. Beside
that, making a mirror cell out of a material with as similar of thermal
expansion to the mirror substrate helps maintain stabilty and reduce the
stress on the adhesive (which also gets applied to mirror). Invar or a
low expansion ceramic is usually the best choice for a mirror mount
though they're expensive. Cast iron has one of the lowest thermal
expansion coefficients of common inexpensive metals and it's easy to
machine. Aluminum is about 2.5 times worse.

A mirror doesn't have to be glued at all. It can can be accurately held
in alignment with as few as three hard points. For a small disk mirror
standing "on edge" a three point mount on the mirrors circumference,
centered on the edges (none on the back) will provide minimum
distortion if the pressure on the points is moderate.

Even the strongest metals and low expansion glass aren't infinitely
rigid. In fact they are quite elastic over a small range of motion. A
thin layer of adhesive can work as a dampener to reduce vibration.
Whether glue is used or not you have to deal with flexibility in the
system. Mirrors will only vibrate if subject to variable force, such as
shaking the table or air currents.

(From: Joe Gwinn (joegwinn@mediaone.net).)

I would add that it's a bad idea to make the silicone rubber too thin,
because the thinner the rubber layer the higher the strain in the rubber
as the temperature changes and the glass moves relative to the mount (made
of aluminium?). Even the difference between daytime and nightime inside a
building can do it. If the strain is too high, the rubber will soon fail,
and the mirror will be able to move around, or even to fall off. The
larger the mirror diameter, the thicker the silicone layer must be. That
said, silicon rubber will tolerate 200% strain (pull to double the length)
acutely, and perhaps 20% for long periods and/or many reversing stress
cycles, if memory serves. See the datasheet and application notes for the
details.

I found this out the hard way fifteen or twenty years ago, when we had
polycarbonate tops just popping off of an instrument with an aluminium
chassis, despite the fact that to pull the top off at first took hundreds
of pounds of applied force. The low-tech solution was little pieces of
toothpick embedded in the wet rubber before pushing the top down into
place, maintaining a minimum rubber thickness.

On the other hand, if the rubber is too thick, then the mirror will be
able to flop around too much, so there is an optimum thickness, but the
optimum will be quite broad.

I don't know the properties of the 3M tape, but I would guess that it too
can handle lots of strain.

Definitions:

Stress -- The force applied to a material. Units are pounds per square
inch or the like.

Strain -- The resulting physical distortion of the material subjected to
stress. Unitless, expressed as a fraction. For example, if the strain is
1% (0.01), the length changed by 1%.

Reversing stress -- Where the force on the material alternates between
compression and tension. If one bends a beam back and forth, the material
near the top and bottom of the beam will suffer reversing stress.
Likewise, the rubber between a glass mirror and an aluminium back as the
temperature cycles. This matters because reversing stress causes much
more material fatigue than non-reversing stress (where the sign does not
change), causing material failure that much sooner.

(From: Zane (zanekurz@sansnetcom.com).)

If you go the flexible epoxy route, you can use soft brass shim stock as
spacers between the mirror and plate to get the glue pads the same
thickness. As mentioned, a number of small pads spaced around is a good
way to go.

You can practice getting the glue pads to look the way you want by using a
piece of plate glass on a metal plate similar to your mount. You can then
get a measure of exactly how much glue to put down per pad, as well as
check the strength of your bond.

There are many times where a solid mechanical means of blocking the beam
is needed at various places in an optical system. This may be to satisfy
safety requirements, for selecting colors, etc. Where speed is not a
concern, several simple low cost reliable approaches may be used.

One very common method is to use what's known as a rotary solenoid. This
device looks like a small motor but its shaft only rotates through a fixed
angle (usually 90 degrees) when power is applied. By attaching a "flag"
to the shaft, activating the solenoid can be set up to open or close the
beam port within a fraction of a second. A spring returns it to the
deactivated position when power is removed. Limiting the motion externally
to less than the full angle will improve response time and reduce vibration.
The most widely known manufacturer of rotary solenoids is probably
Ledex.

A small DC motor can also be used like a rotary solenoid if it's rotation
is limited by stops and a spring or reverse polarity is used for return
motion.

Another method is to use the guts of an electromagnetic relay. A "flag"
can be attached to its moving armature to act as a shutter. The advantage
of this approach is potentially higher speed and lower vibration. However,
the amount of motion is generally smaller so beam size may be a consideration.

A company that specializes in laser shutters in particular is
NM Laser Products.
However, if you're an even minimal scrounger, suitable devices can be found
in all sorts of surplus optical equipment, or as noted above, built from
junk parts.

Power Supply Considerations for Home-Built Lasers

Also see the section: Related Power Supply
Information for much more on high voltage and other specialized power
supply operating principles, design, components, and construction techniques.

Note that in this document and the associated laser power supply schematics,
voltages between 110 and 120 VAC Hot to Neutral (220 to 240 VAC between
Hots on opposite sides of the line) may be shown for power in the USA and
other parts of North America. Likewise, 220 to 240 VAC may be shown for power
in Europe and elsewhere. Where some other voltage is used (such as 100 VAC in
parts of Japan), it will be ideentified explicitly.

Several types of power supplies are used for these lasers (more than one type
may actually be applicable):

AC. A high voltage transformer supplies an alternating voltage (at the
power line frequency - 50 or 60 Hz) to a pair of electrodes roughly at each
end of the laser tube. The amount of voltage/current/power is usually
controlled by using a Variac (variable autotransformer) to power the system.
There are usually no other components beyond possibly a ballast (series
current limiting) resistor.

Luminous tube (neon sign) and oil burner ignition transformers are the most
common types, are simple to use, and relatively easy and inexpensive to
obtain. These typically produce between 5 and 15 kV at 10 to 60 mA and are
internally current limited. The implementation uses a loose coupling with
a magnetic shunt to provide current limiting. For some types like the oil
burner ignition transformer I tested, the behavior is similar to that of a
series resistor that limits current to the maximum specification when the
output is shorted. So, you don't get both the rated voltage AND the rated
current at the same time. Larger neon sign transformers may be constructed
to act more like constant current sources up to nearly their rated voltage.
The input VA rating will probably be roughly equal to the OUTPUT open
circuit voltage times the OUTPUT short circuit current. Unless corrected
(usually with a parallel motor run type capacitor), their power factor when
the output is open circuit will be very low (e.g., .2). Check the
transformer's nameplate - The VA rating divided by your line voltage is the
current your electrical outlet will have to provide (though actual wattage
used depends on output current).

The following applies only to conventional "iron" transformers, not
the electronic type - no funny connection arrangements are possible with
those.

Identical models of these transformers can be paralleled to obtain higher
current than a single one alone can provide. If they are both centertapped
or case connected at one end, the cases should be connected together and to
Earth (Safety) Ground. Just make sure you get the line phase correct!
Don't assume the input terminals are wired the same. Connect one pair of HV
terminals together and wire up a small (e.g., 1/8") gap between the other
pair. If the line phasing is correct, there should be no arc even at full
line voltage. Or, use a Variac to power them and a multimeter to check
between unconnected HV terminals - there should be negligible voltage
between them (a few volts at most) with an input of say, 10 VAC. If the
phasing is incorrect, you will get 2 times the normal output voltage of each
transformer - which you won't be able to miss!

The only series connection of neon sign transformers that is acceptable is
if your models are connected to the case at one end (not the centertap).
Then, connect the AC input out of phase (opposite of above when paralleling)
and tie the cases together and to Earth (Safety) Ground. Best performance
will be achieved if the current ratings are the same but nothing disastrous
will happen if they are not matched.

Check demolition companies, salvage yards, neon sign shops, etc. They
sell old transformers at low prices since a guarantee for long term
reliability cannot be provided - but you really don't care unless your
laser is to be run for years on end. Oil burner types will be totally
free from HVAC contractors - but you will probably have to take the entire
smelly, oily, icky oil burner away as well!

And, no, you don't want to build your own even if you own a wire factory. :)
For a 12 kV transformer, I figure about 400 turns for the primary and over
40,000 (!!) turns of really really fine wire for the secondary. This is all
carefully wound in multiple insulated layers on the special core and then
the entire affair is fully potted to prevent corona, arcing, and all those
other undesirable things that high voltages would do if undisciplined.

DC. The output of a high voltage transformer or high frequency inverter is
applied between a pair of electrodes roughly at each end of the laser tube.
Current through the tube may be controlled by adjusting a Variac as above
and/or by providing an appropriate ballast resistor (as with a conventional
HeNe laser power supply - see the chapter: HeNe
Laser Power Supplies).

The output of luminous tube and oil burner ignition transformers can be
rectified and used to charge high voltage capacitors. However, both the
rectifiers and capacitors must be rated for the voltages involved.

Rectifiers: For a capacitive load, the Peak Reverse Voltage (PRV) rating
must be greater than 2 * 1.414 * VRMS of the transformer for a basic half
wave rectifier circuit or half this for a bridge. (Nearly all these
power supplies feed a capacitive load of some sort even if there are no
actual capacitors in the circuit - laser tubes and wiring act as
capacitors!) This must take into account the maximum voltage even if you
are using a Variac that can supply 140 VAC or greater into the transformer
from a 115 VAC line.

Capacitors: Voltage rating must be greater than the peak of the input
again accounting for higher than normal line voltage from a Variac. See
the section: Standard and Custom HV
Capacitors for more information.

Pulse. A DC power supply charges up an energy storage capacitor which
either discharges between electrodes in the laser tube (or flash tube) when
its voltage rises high enough, when the pressure goes (is pumped) low
enough, or when the gas in the tube is ionized by an external trigger
generator like that of a common electronic (xenon) flash.

Timing may also be provided by mechanical means - a rotating switch or
commutator arrangement feeding the outputs of multiple high voltage
capacitors to the laser tube in sequence like an automobile engine
distributor.

RF. Electrodes or coils outside the laser tube are excited by what is
essentially a radio transmitter power oscillator. Although the first HeNe
laser used this approach, RF excitation doesn't produce a high enough
intensity of the discharge where it is needed - in the center of the tube
bore so it is usually not appropriate. However, it may be possible to use
RF excitation with wide bore designs like that of the CO2 and HeHg lasers.

For additional suppliers (both commercial and private) of the parts needed to
construct these sorts of high voltage power supplies, see the chapter:
Laser and Parts Sources.

As noted above, neon sign or luminous tube transformers do not behave like
ideal power transformers - or even real world power transformers. They are
designed specifically for the needs of gas discharge tubes, which want to see
a current based on their diameter and gas fill, but independent of tube
length (and thus voltage drop). Therefore, a constant current source is
required. The magnetic design of the core and windings very cleverly provides
a decent regulation characteristic and protection from momentary (at least)
short circuits.

Open circuit voltage - This is the kV rating on the nameplate. A 12 kV
unit will output about 12 kV when driven with the nominal AC input voltage
and frequency with no load connected. The high peak voltage (actually almost
17 kV for this example - 1.414 * 12 kV) provides the means of initiating the
discharge on each half cycle since the negative resistance behavior of a long
gas tube - like a helium-neon or argon ion laser - requires a high voltage to
start and a lower voltage to run.

Short circuit current - This is the mA rating on the nameplate. A 30 mA
unit will output about 30 mA into a short circuit when driven with the nominal
input voltage and frequency. The output current supplied by the transformer
will be relatively close to this value for a good portion of its output
voltage compliance range.

Reactive power - This is the VA rating on the nameplate and for a typical
transformer is very close to the kV * mA rating.

Input current - Varies from approximately .1 (no load) to 1 (short
circuit) of the 'A' from the VA rating. For the 12 kV, 30 mA unit I tested,
this was about .4 A to 3.1 A respectively (at a line voltage of 115 VAC).

Power Factor - Probably not specified but typically quite low (unless unit
has built in correction). Partial correction using a parallel capacitor is
possible but its uF value may need to be determined under expected operating
conditions since PF depends on load.

Regulation - Between an open and a short circuit, the core and winding
construction results in a quasi-constant current characteristic over much
of this range. I did a test on a 12 kV, 30 mA transformer at reduced voltage
(I didn't have any way of providing a variable load at full output so I used
a Variac to set the no load output voltage to 1,000 VAC):

R was equal to 392K ohms (I have a bunch of them). So, for loads resulting
in between about 1/2 and rated output voltage, the current changes by less
than 30 percent - which isn't bad for something without any silicon! The
Thevenin equivalent for this transformer over the range of 0 to 350 V or 2.1
to 1.8 mA would be 1.129M ohms fed from a 2.45 kV source (remember, this was
done at reduced voltage. At nominal input this would be equivalent to almost
30 kV). These measurements were very approximate. I expect that behavior at
full voltage (and its associated current) won't be quite the same (actually,
it will probably be better) but this demonstrates the general idea.

Another thing to note is that when a NST is used with an unbalanced load such
as only one half of the centertapped secondary being used or when feeding a
half wave rectifier, its characteristics may not be what are expected based on
experience with a normal power transformer. When half wave rectified, for
example, the output current won't be just half of the full wave current but
may even be greater (2.9 mA compared to 2.1 mA for the short circuit
case, above). Presumably, this results from the fact that on half the cycle,
the core is seeing the full output voltage. While such characteristics may
not be inherently bad, it may not be easy or even possible to make predictions
so testing would be needed for each particular situation.

You can estimate the voltage rating of an unlabeled NST by running it as above
on a Variac at say, 5 percent of line voltage, and measuring its output
voltage. Then, multiply by 20. To determine the current rating, connect the
output directly to an AC current meter. To be cautious, start at low input
voltage and go up to full line voltage (since the NST should be current
limited).

WARNING: The current test assumes a current limited neon sign or oil burner
ignition type transformer. Doing this on a normal power transformer will
probably result in a blown fuse/popped circuit breaker, blown meter, or both!

Here is some additional information on the electrical characteristics of neon
sign transformers (NSTs) including power factor issues and correction. A 15
kV, 60 mA unit is assumed - adjust the numbers for whatever size you have.

(From: John De Armond (johngd@bellsouth.net).)

Let me answer several questions at once. First, a 15 kV, 60 mA transformer
will produce 60 ma almost up to its rated voltage. The transformer is
designed to be a constant current device, to supply whatever compliance
voltage is needed to push the 60 ma through the load. The 60 ma is nominal
short-circuit. All magnetic transformers made for use in the US are designed
for continuous use at no more than 80% of the short-circuit current.

I never actually sat down and plotted it out but I do know this: With 1 foot
of neon tubing on a transformer (about 500 volt drop), it drives 60 mA. With
over 60 feet of tubing on the tranny (more than specified), it still outputs
about 50 to 53 mA. That's fairly constant current.

That said, a NST will NOT survive long if asked to supply full voltage at full
current. It is designed to drive a gas discharge tube. The characteristic of
a gas discharge tube is that it takes a large amount of voltage to ignite the
discharge and then the voltage falls to a fraction of the starting voltage to
sustain the discharge. Thus the high dissipation occurs only for a short
period of time in each half cycle. On a scope, this looks like a sharp spike
followed by a level, square wave form for the rest of the half cycle. This
sequence occurs 120 times per second.

Regarding volt-amps and watts. You left off the critical part of the equation.
While for DC and 100% resistive AC loads, the formula is W = E * I, for
typical loads that include some capacitive or inductive reactance, the
equation for power is W = E * I * cos(theta) where theta is the phase angle
between the voltage and the current waveform. Volt-amps is simply E * I and
includes both the real component and the reactive or out-of-phase component.
The term "power factor" is simply cos(theta). In a pure inductor or
capacitor, the current is 90 degrees out of phase with the voltage, cos(theta)
= 0 and so no real power is dissipated. This even though the cap or inductor
is drawing amps that can be measured. For an inductor, the current lags the
voltage by 90 degrees and for a cap, the current leads the voltage by 90
degrees. If one measures the current to a reactive device (cap or inductor),
the measured current will be the quadratic sum of the real (in phase) and
imaginary (out of phase) current.

An AC wattmeter measures real power. In other words, it compensates for
cos(theta) Wattmeter test instruments are available in a form that uses a
clamp-on current probe to measure the current and a physical connection to
measure the voltage. These will typically display volts, amps, watts, VARs
(volt-amps reactive) and PF. They are also expensive. For the experimenter,
an ordinary utility power meter is an accurate, if less convenient
alternative. Widely available surplus (C&H Sales and others), the meter is
accurate typically to better than 2% over a 10:1 range. the numbers on the
front register watt-hours while the RPM of the meter wheel measures watts.
The Kh factor printed on the meter face is how many watt-hours each revolution
represents. Typically 7.2 for residential meters. Simply count the turns
over a measured period of time, multiply by Kh and divide by the measured
interval in hours to get watts. I have a recording watt-hour meter that was
equipped with a photo-interruptor to count revolutions. One can easily add
one to any meter using a reflective photo-interruptor to look at the black
flag on the meter wheel. (Do NOT attempt to drill a hole in the dial for
counting - that will destroy the calibration.)

The PF of a standard neon transformer is very low, typically in the range of
0.2 to 0.4 lagging. This is why the VA ratting is much higher than the watts
that can be supplied. That means that the transformer draws more than twice
the current required to supply the output wattage. This reactive current,
called "wattless current" in the slang, can be countered by supplying an equal
amount of leading phase angle wattless current. A capacitor does that. A
motor run capacitor is the proper type which can handle the continuous duty.
To compensate a tranny, simply start adding capacitance while watching the
amperage draw from the line. When the draw is at the minimum, the capacitive
reactance is equal to the inductive reactance, the PF to the line is 1 and all
is well in paradise! A 15 kV, 60 mA tranny will need about 160 uF of parallel
capacitance. This varies with secondary load so one must measure but that's a
starting point.

Note that the full current (wattless + real) is still flowing in the circuit
between the cap and the tranny.

This technique is widely used in neon sign work. It will allow twice as many
transformers to run on a given branch ampacity or else it will allow lighter
wire to be run to a given load. For fully enclosed transformers (HV terminals
are inside the box), there is enough room for the cap inside.

(Portions from: Mark Dinsmore (dinsmore@ma.ultranet.com).)

There is a very good analysis of the design of neon sign transformers in:

Among the multiple pages of design information is a graph of the load line
of a typical magnetic (iron) neon sign transformer.
Neon Sign Transformer Electrical Characteristics is
redrawn version of this diagram. Note that for laser tubes as well as neon
signs, factors other than tube length are important in determining discharge
voltage. These include tube diameter, gas fill, and pressure.

I don't know if the following is a newer edition of the same book, but it
might be an alternative source of a lot of additional information on these
topics:

The discussions below relate to neon sign transformer availability and size,
testing, use of larger (pole pig) transformers where more power is needed, and
reasons NOT to consider microwave oven transformers in these applications.
However, microwave oven transformers can be modified for other uses like
powering the filaments of argon/krypton ion laser or thyratron tubes. See the
section: Rewinding a Microwave Oven Transformer
for use as a Low Voltage Filament Supply.

(From: Jason Freeburg (egraffiti@iname.com).)

A used neon sign transformer should not cost more than $20 or so. Find a
neon shop in your area. They usually have the used ones stacked up
somewhere and will sell cheap. The 60 mA models are usually somewhat
cheaper than the 30 mA type if you buy them used from a neon shop because
they are really too hot (e.g., provide too much current) for running neon
and they cause staining and premature burnouts. It all depends on the
particular shop you go to. I don't suggest buying new for something like
this, the performance will be the same but the price much higher. A new
15 kV, 60 mA transformer lists for about $80.

BTW, the best name to look for in neon sign transformers is
France. These things are ruggedly
built and will take a lot of abuse without dying. The name to avoid is Actown
- their transformers are wimpy and usually don't deliver the rated current.

(From: John De Armond (johngd@bellsouth.net).)

Testing of a used neon sign transformer is pretty easy even without test
equipment. These normally fail with a secondary short and all that does is
(slowly) cause them to overheat and let all the magic black goo run out.

Wire the transformer primary up with a 3 wire grounded cord (green to the
case!), plug it in, and see if you can use a plastic-handled screwdriver to
draw an arc from each insulator to the case. If that works and they don't
make any funny noises, they're probably OK. The grounded cord will also weed
out any trannys with a primary short to ground.

(From: Sam.)

While some of the discussion above might suggest that you should run right out
and corner the market on old neon sign transformers because newer ones won't
work properly for home-built lasers, you can relax. The nice simple iron
current limited type aren't going to disappear overnight - there will be
plenty of piles of used transformers in neon sign shops for years, if not
decades, to come!

(From: John De Armond (johngd@bellsouth.net).)

The glory days of neon transformers for experimenting are coming to an end.
UL and the NEC have conspired to rewrite the code to require secondary ground
fault protection to built into new transformers. These protectors trip the
transformer if the secondary current is unbalanced or goes to ground. My
testing of the units on the market show them to be very sensitive to spurious
currents, particularly RF (as will exist in a gas discharge tube at higher
pressure). It must be built in and be inaccessible to the installer. This
means potted in tar. I've X-rayed a couple to try and figure out where that
"special place" would be to drill a hole to disable the devices but since most
of these are still at least partially hand-assembled, the parts placement
isn't accurate enough to make a template. Used transformers will still be
available from sign shops as they are replaced with SGFI (Secondary Ground
Fault Interrupter) transformers but then the supply of unprotected ones will
go away. Therefore basing plans on neon transformers could be shortsighted.

As for more power than the conventional 15 kV, 60 mA neon sign transformer:

The highest power leakage-flux limited (neon) transformer that is available is
actually called a cold cathode transformer with a maximum rating of 15 kV, 120
mA. These are available from neon suppliers but are not common and will
usually have to be ordered.

Beyond that is the common domestic pole pig. That is, a pole-mounted utility
transformer. For my neon bombarder, I have a 25 kVA, 15 kV pole pig driven in
reverse from the 240 volt main. A modified Miller welding machine hooked in
series with one leg is the current limiting choke. As configured it will
produce 2 amps at 15,000 volts!! The pig itself will produce over 10 amps
before it saturates if you can drive it. :-) Utility transformers have a
service factor of at least 2.5 so it can do that all day.

(From: Sam.)

And, before you ask, while microwave oven transformers might seem to be useful
for home-built CO2 (or other) lasers, I have three problems with recommending
them:

They are definitely most extremely positively lethal (is that enough?) if
your body comes between the HV and its return. They may be able to source
an AMP or more without even thinking about it. By design, the frame serves
as the return for the HV which complicates things further.

While they may be current limited (there is typically a magnetic shunt
between the primary and secondary windings), the maximum current is likely
to be enough to pop fuses or breakers if its output is shorted. (This is
common from failure of the HV capacitor in a microwave oven - microwave
oven's internal 15 or 20 A fuse blows instantly.) This apparently isn't
true of all of these transformers but probably of the better built ones,
ironically. And, there will be significant fireworks as well!

For most gas lasers, there will have to be serious ballast resistance to
stabilize the plasma discharge and prevent overheating of the laser tube.
Neon sign transformers provide most or all of this ballast resistance by
the current limited design using a loose magnetic coupling.

For all home-built lasers except the CO2 laser, the use of microwave oven
transformers is something like killing flies with H-bombs - just a bit of
over-kill (possibly literally). For higher power CO2 lasers, they could
make sense based on their wattage but in my opinion, the very significant
disadvantages more than outweighs this possible benefit.

As for ballast resistance, the discharge characteristic of small commercial
CO2 laser tubes is supposed to be something like -200K ohms. So, you need at
least 200K (actually figure 30 to 50 percent more to be on the safe side) of
positive resistance to maintain stability - this is automatic with the neon
sign transformers whose internal equivalent series resistance is typically
250K or more. It may be possible to use a high voltage capacitor (like the
one in that microwave oven) to limit current - yet another object to zap the
careless! Home-built CO2 lasers with wide bore tubes probably have a negative
resistance that is a lot lower but you will still need some external ballast
resistance since there is essentially none provided by the transformer itself.
In addition (as if this isn't enough?) the relatively 'low' voltage of these
units compared to neon sign transformers means either (1) that starting of
some laser tubes may be a problem even if the voltage is adequate for operation
and (2) you may be tempted (shiver!) to put 2 or more microwave oven
transformers in series. Two *could* be used with their returns tied together
and driven out of phase to create a centertapped arrangement like that of the
typical neon sign transformer.

Please don't consider any of this unless you have lots of experience working
around high voltage high power equipment! Microwave oven transformers
significantly exceed the lethality factor of pole pigs if for no other reason
than the physical setup is likely to be closer to something out of a bad Sci-Fi
movie than a well designed, safe, protected (in a relative sense, at least)
system! At least the pole pig *looks* suitably dangerous due to its size and
impressively large porcelain insulators! And, no, I don't recommend CT scanner
X-ray generator transformers either. :)

The typical microwave oven power transformer consists of a primary of 100 to
200 turns (for 115 VAC power), a high voltage secondary of several thousand
turns, and a filament winding of a few turns of thick wire that goes directly
to the magnetron filament terminals (possibly with a tap to connect to the
other HV circuitry). Replacing the filament winding with one of a suitable
number of turns to drive your tube is usually a simple matter as all the
windings are likely on separate sections of the core and disassembly is
straightforward, though tedious. (However, this will probably have to be done
without removing the core as it is likely of welded construction.)

WARNING! EXTREME DANGER: The HV winding is deadly and one end is grounded to
the core. If you aren't going to be using the high voltage winding (which is
the desired state of affairs!), it is best to remove it entirely by a
combination of hack saw, chisel, pry bar, and explosives. :) Do make sure your
health insurance is paid up and you know the directions to the nearest
emergency room and/or the number of the your local ambulance service - some of
these techniques can result in personal injury. :(

If you don't remove the high voltage winding, make sure adequate insulation
(e.g., electrical tape, Plexiglas shields) is added to absolutely prevent
contact.

To drive an argon/krypton ion tube (2.5 to 3 VAC) or thyratron filament (5
to 6.3 VAC) will require only a few turns of heavy wire. Use a wire size of
at least #12 for a current of 15 to 25 A, #15 for 10 to 15 A, and #16 for
5 to 10 A. The specific number of turns is best determined by experiment
(dependent on the actual number of turns in the primary winding, the voltage
drop in the wiring, and your exact line voltage) but will be between 1 to 2
turns per volt RMS of output (for 115 VAC nominal line voltage). Where a
centertap is needed (i.e., for an ion tube filament supply), it is probably
best to bring out each half separately and connect them externally.

Whatever you do, make sure there is no possibility of this filament winding
coming anywhere near the high voltage winding if you haven't removed it!
Also, note that some applications like a HV pulser require that the filament
winding be insulated to withstand the full output voltage to ground - maybe
15 or 20 kV or MORE!

Nearly all the power supplies for the home-built lasers as described in "Light
and its Uses" and elsewhere use a a variable autotransformer (Variac is one
particular brand name) to adjust voltage and/or current.

WARNING: A Variac is NOT an isolation transformer and provides NONE of its
safety benefits! Due to the power requirements of many of these lasers, it
is not really practical to use a small isolation transformer for testing.
However, make sure you understand and follow the information provided in:
Safety Guidelines for High Voltage and/or Line Powered
Equipment.

The most common variety (at least that you are likely to use) connects to the
115 VAC line and outputs either 0 to 115 VAC or 0 to 140 VAC depending on how
it is wired. There are both open frame 'bare bones' and fully cased units.
The latter may include a line cord, on/off and possibly low/high range switch,
power indicator, and a 3 prong grounded outlet. If you have an older unit with
only a 2 prong cordset and outlet, I would recommend replacing them with a
heavy duty grounded cordset and 3 prong grounded outlet.

There are also models that will accept 115 VAC and output upt to 0 to 280 VAC -
useful for powering or testing 220 VAC (e.g., European or high power laser)
equipment. There are three-phase models as well (but you need a three-phase
power feed for them to be of much value!).

To determine what size you need, check the full load input requirements of the
equipment you will be powering. Neon sign transformers provide this
information on the nameplate; regular transformers may not; in this case,
estimate the input volt-amp (VA) requirements by adding up the full load
secondary VA ratings and multiply by 1.10 to 1.15 to account for transformer
losses.

Sam's Rule #61453: Your lab can never have too many Variacs!

Sam's Rule #61454: If you only have a single Variac, it should be LARGE!

Although prices for new Variacs are stratospheric (figure $100 and up for
anything large enough for home-built lasers), they can often be found at
hamfests and high-tech flea markets, offered for sale in various USENET
newsgroups like sci.electronics.equipment, and sometimes even turn up at
regular garage sales (which is where I have gotten 3 of mine). In most cases,
unless there has been a total meltdown, a Variac will survive just about
forever with at most the need for a new carbon brush. These things are very
robust so getting stuck with a bad one isn't all that likely (but not
impossible). Also check the surplus and private parts sources listed in the
chapter: Laser and Parts Sources.

See the next section for Variac wiring.

For adjusting the power to a transformer or other inductive load over a
possibly slightly restricted range, there may be an alternative to an
expensive, heavy, Variac. Some relatively simple modifications to a common
light dimmer will permit it to substitute for a Variac for some applications.
This is detailed at:
A Two-Into-One
Homemade Neon Dimmer. The idea is to get around the inductive lag in
current flow that confuses a normal dimmer and may cause it to blow up.
However, the voltage and current waveforms are going to be nasty with this
approach - which may not matter but is something to keep in mind. If you do
this, make sure there is a fuse for this circuit alone - some failures of the
triac or its trigger circuit can result high current DC through the primary of
your transformer which is not something you want to experience.

Another option, more applicable to lower power equipment, is the use of a
fixed or variable power resistor (rheostat) in the primary circuit. The
problem with this is that all three of the following conditions must be
satisfied:

Its power rating must be adequate. For a variable resistor, this means
at least 1/4 of that of the controlled equipment - possibly more if it is
non-linear.

Its current rating needs to be at least equal to that of the controlled
equipment maximum rating.

It needs to be safely mounted somehow and WILL dissipate a lot of heat!

In addition, with the rheostat, the sensitivity of the control to small
changes in its setting will be greatest at one end of the rotation.

However, for a small fixed reduction in current/voltage, a power resistor may
be a good idea as the dissipation will be modest and it can be mounted inside
the equipment case out of harm's way (as long as it is adequately cooled).

WARNING: Direct connection between input and output - no isolation since the
power line Neutral and Ground are tied together at the main service panel
(fuse or circuit breaker box)!

CAUTION: Keep any large transformer of this type well away from your monitor
or TV. The magnetic field it produces may cause the picture to wiggle or the
colors to become messed up - and you to think there is an additional problem!

Note: the 'Power LED' circuit is soldered directly to a winding location
determined to produce about 6 VAC.

Wiring is straightforward if you have acquired a bare unit (the following
assumes a 115 VAC line, the extension to 230 VAC should be obvious):

Decide on the output voltage range and direction of rotation (clockwise
should always be used to increase output as far as the user is concerned but
depending on mounting, actual direction of the shaft with respect to the
body of the unit may be either way):

For 0 to 115 VAC output, Hot and Neutral go between the ends of the
winding with Neutral at the terminal where the wiper will be when you want
the output to be 0 VAC; The output goes between Neutral and the wiper.

For 0 to 140 VAC output, Hot is moved to the tap about 20 percent away
from the terminal where the wiper will be when the output is at 140 VAC; The
output goes between Neutral and the wiper as above.

For some (mostly small) Variacs that do not have intermediate taps, it may
still be possible to add a tap to permit 140 VAC operation. However, this
will reduce the number of turns on the primary and could lead to overheating
from core saturation if the design is marginal. Most of these are
overdesigned and this shouldn't be an issue. But, nonetheless, if you try
this, monitor the temperature of the unloaded Variac for, say, an hour
to make sure it doesn't get excessively hot.

Include a primary-side power switch and power-on indicator lamp. In the
old days, a neon lamp would be used (e.g., NE2H with a 47K ohm resistor).
Nowadays, neon lamps may be hard to find. An alternative as suggested above
is to add a tap on the Variac winding at about 6 to 10 VAC and use an LED and
current limiting resistor.

Provide fuses for both input and output. They should be the same
rating as the Variac. The fuse for the input protects against primary side
shorts. The one for the output protects the Variac winding from excessive
load. Commercial units may only have one fuse but fuses are inexpensive and
the added protection won't hurt.

Use an adequately rated grounded cordset and mount everything in a well
insulated box. The Variac frame and box (if made of metal) should be
grounded.

Here is more than you ever possibly wanted to know on the subject. For a
simplified discussion of this topic, as well as for suppliers of commercial
high voltage rectifiers (which would make this exercise unnecessary), see the
section: Standard and Custom HV Rectifiers.

Laser power supplies usually require high voltage power supplies. Often, the
voltage is greater than the capabilities of commonly available components such
as rectifier diodes.

The usual solution to this problem is to connect more than one device in
series. Unfortunately, in the real world, components are not matched closely
enough for this simple trick to work. The purpose of this article is to
explain how to design series-connected rectifier strings, which will operate
reliably in spite of component mismatches and tolerances.

Although the emphasis is on rectifiers, the principles are also applicable to
switching elements such as thyristors, bipolar transistors and mosfets.

A set of rectifiers connected in series will be referred to as a 'string'.

The first task is to compute the maximum reverse (blocking) voltage that the
rectifier string must withstand. This depends on the type of rectification
which is required (full or half) and the transformer output voltage.

The following examples assume that the transformer output is 1 volt RMS.
Simply multiply everything by the appropriate factor for other voltages.

For example, for a full-bridge rectifier used with a 15 kV transformer, each
of the 4 diodes in the bridge will need to withstand 15 kV * 1.414 = 21.2 kV.
This is a minimum value. An additional safety factor of 15% should be
provided to account for mains overvoltage, giving a design point of 24.4kV, or
say 25 kV.

Commonly available rectifier diodes, such as the 1N4007, are rated for 1kV,
thus (in theory) at least 25 of these will be required for each of the four
strings to make up the full bridge rectifier. This is not as bad as it
sounds, since packs of 100 1N4007's can be obtained for under $5. It is
tedious to perform all that soldering, but the alternative of buying
ready-made rectifiers of the required voltage will be much more expensive.

Having selected a peak reverse voltage rating for each string, the number of
devices in the string must be computed. This is not as simple as dividing the
total voltage (Vs) by the rating of each diode (Vd), since a reverse leakage
current will always flow. This reverse current causes an imbalance in the
voltage seen by each diode, with the highest voltage impressed across the
'best' device, and the lowest across the most leaky device.

The time honoured way of dealing with this is to add resistors in parallel
with each diode in the string. This will increase the 'leakage' current,
swamping out the variation between the diodes. The price to pay for this is
wasted power, and obviously the cost of the resistors. To mitigate the
former, the highest value resistance should be used which affords the proper
voltage sharing.

It is impossible to completely balance the voltage in this way, since
resistors have their own tolerance. Thus the only alternative is to add extra
diodes in the string, to allow for the imbalance. There is a tradeoff to be
made: increasing the string number will reduce the wasted power, but increase
the total cost.

To compute the string number and the parallel resistance values, the following
information must be known:

Imax - The maximum reverse leakage current of the diode. Manufacturers
usually specify this at the rated reverse voltage and maximum junction
temperature.

Imin - The minimum reverse leakage. This is not usually specified so, to
be conservative, it is assumed to be zero.

Vd - The diode reverse voltage rating.

Vs - The string voltage.

a - The available resistor tolerance. In the following, this is expressed
as a fraction e.g. 0.05 for +/-5% tolerance.

n - The string number (i.e. number of diodes in the string). This may be
initially set to, say, 10% more than Vs/Vd, then increased or decreased as
required (based on allowable wasted power and other considerations).

And the total power dissipated by the entire string of diodes and
resistors is:

Vs * Vd
Ptot = -------------
R * (1 - a)

These values may be reduced somewhat, since the string will not be
continuously blocking the maximum reverse voltage. For sinewave
rectification, the above figures may be divided by 2 to 4.

Note that resistors have a voltage rating as well as a power rating.
Typically, the voltage rating is 300 or 600 V (600 V for 1 W types, 300 V for
1/4 W). Values above 1M are hard to obtain in tolerances under 5%.

Since 1N4007s are rated for 1 kV, each parallel 'resistor' should actually be
a series combination of two resistors, unless you can locate special
high-voltage resistors.

Given that 1N4007's, with a maximum leakage of 5 uA, are to be used for a
full-bridge 15 kV RMS rectifier, design a rectifier string. Resistors of 10%
tolerance to be used, since we expect to use two 5% resistors in series.
Additional specs: Vs = 25kV (from above), Vd = 1kV, Imax = 5uA, a = 0.1 (10%
tolerance).

Using an initial guess of n = 30 (i.e., 20% more than the theoretical 25
required):

Negative values for R mean that the initial guess for n was too low. As it
turns out, the culprit here is the loose tolerance of the resistance. We can
either tighten the resistance tolerance, or increase n. Trying for 2%
resistors, the result becomes: R <= 27M.

This is a high resistance, which is good, but practically unobtainable in 2%
tolerance. This is the point at which a decision needs to be made. We can
either

Reduce the string number, and retain 2% tolerance, hoping to get a value
under 2M.

Increase the string number and go back to 10% tolerance.

Use a lower valued resistance anyway and wear the power loss.

The first option seems more cost-effective, so we will try n = 27. This gives
R <= 7.7M. Still too high, but reducing n to 26 requires such a low
resistance that too much power is wasted. At this point, we cut our losses
and decide to settle on 2M (i.e., two 1M, 1% resistors in series). The worst
case power dissipation of the combo is

.5 * 10002
Pd = --------------------
2x106 * (1 - 0.02)
= 0.256 W

Each resistor will only see half of this, which is very small. This is good
for long life and stability. We can't use 1/4 W metal films, unless they are
rated for at least 500 V. 1/2 W metal films would normally be OK.

Having worked all of this out, the complete bill of materials for the bridge
rectifier is:

108 1N4007s.

216 1M, 1%, 1/2 W metal film resistors.

All this should cost under $50.00.

Special high-voltage resistors are available, which cuts down on the
inconvenience (if not cost) of series combinations. For example, Philips has
the VR25 series of 1/4 W 5% metal glazed in the range of 1.2M to 10M, and the
VR37 series of 1/2 W 5% in the range 1.2M to 33M. The VR25's are rated for
1,600 VDC and the VR37's for 3,500 VDC.

If you are thinking of 'cheating' by hand-selecting matched resistors, then be
aware that this is not good for long term reliability. Even if you select two
resistors that are matched within 1% on day one, then after time they will
more than likely drift apart. This is because they are stressed by high
voltage, which is a well-known cause of long-term drift.

The above considerations apply only to DC or slowly varying voltages such as
sinusoidal mains (50/60 Hz).

When higher frequencies are involved, it is also necessary to account for
transient phenomena such as diode reverse recovery charge. You can ignore
this section if you are only concerned with low frequency sinewave circuits
(<200 Hz). Note: a low frequency square wave actually has high frequency
components, so this section may still be relevant.

When a current is flowing through a diode in the normal (forward) direction,
then the voltage is suddenly reversed, the forward current will rapidly
decrease, go through zero, then actually reverse before finally snapping back
to the leakage level. The rate of change of current will depend on external
circuit inductance.

In effect, the diode conducts current in the reverse (normally blocking)
direction for a short period after voltage reversal. The peak magnitude of
the reverse current may actually be greater than the forward current that was
initially flowing.

This phenomenon is known as reverse recovery charge (Qrr), and is common to
all PN junction rectifiers. Rectifiers used in high frequency circuits are
designed for rapid reverse recovery (i.e low Qrr), and are designated as
'high-speed', 'ultrafast', 'soft recovery' etc. Ordinary 60Hz rectifiers are
not optimised for this, and thus are unsuitable for use above a few hundred
Hz. (Note that fast rectifiers have higher on-state losses and much higher
reverse leakage -- there's always a down-side.)

When using series connection of high-speed rectifiers, Qrr is another variable
quantity which needs to be 'soaked up' by some auxiliary sharing circuit, in a
similar manner to reverse leakage current in the low frequency case. This
sharing is accomplished by means of capacitors which are connected in parallel
with each diode in the string.

To compute the capacitance value, the following values need to be determined:

Qmax - Maximum reverse recovery charge. This is sometimes specified by
manufacturers under various conditions. The value depends on initial forward
current and junction temp. If not directly specified, the manufacturer will
specify the peak reverse current (Irm) and the recovery time (trr). From
these values, you can compute Qrr roughly from Qrr = 1/2 Irm trr.

Vd - The diode reverse voltage rating.

Vs - The string voltage.

a - The available capacitor tolerance. In the following, this is
expressed as a fraction e.g. 0.05 for +/-5% tolerance.

n - The string number (i.e. number of diodes in the string). This may be
initially set to, say, 20% more than Vs/Vd, then increased or decreased as
required (based on allowable string capacitance).

With the high voltages involved, it is important to avoid corona discharge.
Corona causes creation of ozone, which is destructive to the surroundings
(especially rubber and plastics). Corona is exacerbated by sharp points at
high voltages.

To minimise problems, all exposed wiring should be encased in neutral cure
silicone. Don't put silicone over the resistor bodies since it may cause them
to overheat.

Make sure there is an air gap of at least 2 mm per kV between two conductors.
For solid surfaces, the gap should be at least 5 mm/kV since a humid atmosphere
will increase surface leakage.

(From: Mark Dinsmore (dinsmore@ma.ultranet.com).)

Here are a couple of comments that might be pertinent, based on my own
experience.

First, use of carbon composition resistors is highly recommended for HV use.
They are orders of magnitude more robust in the presence of hv transients. I
have put 10kV transients across a 1 watt resistor repeatedly(in Fluorinert
dielectric fluid) with no failure and very minimal changes in resistance.
Allen Bradley makes the devices I am using. However, I don't know if they can
be obtained in the appropriate tolerance.

Second, if the entire rectifer assembly is potted in silicone, mineral,
etc. oil, the issues of HV breakdown, corona, and thermal dissipation are
almost completely eliminated. The oils have a very high breakdown voltage,
making the issues of spacing much easier. The oil also acts as a convective
cooler for the devices, and is very effective in removing the heat generated
in the diodes and resistors.

Knowing the precise values of voltage and current in your power supply is
often useful - sometimes essential. For some lasers, current is the critical
parameter to control to achieve the optimum operating point. Knowing the
voltage across the tube can tell you something about the gas pressure, health
of the electrodes, and so forth.

It is unlikely that you will have or can find exactly the types of meter needed
for each of these lasers. However, any sort of mA or uA meter cn be turned
into a DC or AC voltage or current meter of almost any full range sensitivity
quite easily. This can be a moving coil (D'Arsonval) type or digital panel
meter module. For historical reasons, we call these 'movements' whether they
have moving parts or not. :-)

Note: Where the use of a panel meter is suggested below, if you salvaged the
meter movement from some other equipment or your junk, box, double check the
actual sensitivity. There should be a rating printed at the bottom of the
meter face or the back of the unit like "fs=1mA", "fs=10A", or something
similar. Even though the scale may be labeled with a particular set of units
and values, a series or shunt resistor may actually be needed to adapt the
basic movement to read at that sensitivity. An AC meter may actually use a
DC movement with an external rectifier! It's also a good idea to test the
meter to confirm that it hasn't been damaged due to an overload or just age.

In the discussion below, Im is the full scale sensitivity of the meter
movement and Rm is the resistance of the meter movement.

For the voltmeters, the series resistance has been divided into a fixed
(R1, R3) and variable (R2) for easy calibration. There are some special
requirements for high voltage voltmeters in particular:

Try to select R1 versus R3 taking into account which end, if any, of the
circuit is at ground potential to place the meter itself and the
calibration pot (R2) near ground potential as well.

The series current limiting resistors must be rated to safely handle the
voltage across them as well as their power dissipation. Normal resistors
are usually rated for several hundred V (check the specs) so multiple
resistors in series are required to achieve the required voltage rating.
Alternatively, there are special HV rated resistors designed for this
purpose.

For the current measuring circuits, additional fixed (R1) and variable (R2)
series resistors have been added in addition to the required shunt resistor
(Rs) to allow a convenient selectrion of shunt resistor value (to produce a
1 V drop across Rs for full scale current).

The DC circuits are discussed first. These are generally simpler than those
for reading AC directly and are therefore preferred if a suitable location can
be found where the measurement will be just as meaningful and accurate.

For AC, there are special AC reading meters but these are much less common
than the DC variety. However, where absolute precision and linearity isn't
needed, and an average rather than RMS reading is acceptable, it is a simple
matter to convert a DC meter to respond to AC.

However, while making average measurements of voltage and current in AC powered
circuits is relatively easy and useful setting the operating point and
monitoring overall system behavior, the readings can be quite deceiving when
driving gas discharge tubes. In particular, it isn't possible to just multiply
the I and V values together to compute power input (as would be desirable when
estimating laser efficiency - assuming you have some means of measuring optical
beam power). The AC waveforms are likely to be quite nasty (it is a gas
discharge after all, not a resistor) and will result in significant errors
when computing power using the simple equation: P = I * V.

(From: Terry Greene (xray@cstel.net).)

"Folks should keep in mind that a gas discharge as viewed on a scope looks
sort of like the noise at a rock concert. The only relevant data is the peak
voltage required for breakdown. This is not what you will measure with a
meter. What you will see is a perverted version of RMS voltage which will not
be accurate. It won't even be an accurate RMS reading. It should serve as an
interesting bench mark, but the numbers won't be real. The only way I know of
that you will gather usable info on voltage is to assemble a resistor network
and scope it. Interesting data for design purposes, but not really relevant
to function. I'm not saying you shouldn't put a meter on, just don't
misinterpret the readings."

Precise measurements of power can be performed by integrating I * V over a
complete cycle and multiplying by the number of cycles per second (60 or 50 as
appropriate). As you might expect, this is well beyond the capabilities of the
average multimeter but a piece of cake if you have an instrument designed for
this purpose. :)

The following circuits for AC voltage and current measurements will actually
read the average, not RMS if components values are calculated using the same
equations as for the DC case. For sinusoids, a simple correction can be made
with the calibrate pot. True RMS readings are left as an exercise for the
student!

AC voltage (VACfs): Where VACfs is much greater than .7 V (e.g., 100 V), a
a rectifier diode can be added in series with a DC meter. Ignoring the
voltage drop of the diode, the required current limiting resistor will be
equal to: (Vfs/Ifs - Rm)/2.

Note that since a half wave rectifier is used, the total series resistance
must be half of what it was for the DC measurements.

For high voltages where finding diodes with sufficient ratings is a problem,
use a bridge rectifier - almost any type will do since it doesn't need to
block more than a volt or so. Why? Consider the following:

Since the voltage across the meter movement itself is probably no more than
a fraction of a volt, this is all the bridge has to worry about! There will
be a dead-zone between +/1 1.5 V or so but who cares on a meter that reads
10 kV full scale. Ignoring this voltage drop, the required current limiting
resistor will be equal to: (Vfs/Ifs - Rm).

A blocking capacitor can be put in series with the input to this circuit if
there is a DC offset but it must withstand the full voltage (AC peak + DC)
and have a low impedance at the frequency range of interest compared to the
sum R1+R2+R3.

AC current (VACfs): A series resistor can be placed in the circuit and then
the AC voltage across it can be measured using either of the approaches,
above, or a bridge can be used directly. This time it does need to pass the
full current being measured AND there will be a voltage dead-zone around the
zero crossings of about 1.5 V (however, this won't affect the current
reading linearity. The shunt resistor is equal to: (Im * Rm)/(Ifs - Im).

This approach multiplies the sensitivity of any clamp-on ammeter. I call it a
"reverse current transformer" because it expands the sensitivity of the meter
instead of reducing it, as does a range expander. I've made several of these
and given them out to area neon people and they all love 'em.

The thing consists of an empty Teflon pipe tape spool and shell. 100 turns of
magnet wire are placed on the spool and led out via TV anode wire (rated
40 kVDC). The whole assembly is vacuum-potted in thin clear 2-part Epoxy.

Vacuum potting is easy to do with an old pressure cooker. Remove and plug all
the safeties and install a Schraeder valve in place of the steam regulator.
Mix up the Epoxy in a cup, place the device in it with the tip of the device
exposed to air (to allow air to escape), place the whole thing in the pressure
cooker and draw as good a vacuum as you can. Don't use your high vacuum
equipment - the Epoxy vapors will contaminate it. I use an air-driven venturi
vacuum pump.) After a few minutes of evacuation, slowly release the vacuum.
Air pressure will drive the Epoxy back into the coil. Fully submerge the coil
and repeat. Allow to cure. Attach alligator clips to the leads and the
project is finished.

(From: Sam.)

This same technique can also be used for all those other modules that need to
be potted like custom HV capacitor and diode arrays. However, I is the
vacuum potting really essential for this gadget? While it DOES make for a
really cool looking assembly AND most excellent HV insulation to the outside
world IS needed, there shouldn't be any voltage differences of any
consequence inside the device. However, the additional insurance won't hurt.

(From: John.)

It IS the high voltage to ground you worry about. All high voltage neon
transformers are grounded at the center tap. That means that the current to
be metered can be as much as 7.5 kV above ground, more if there is resonance
in the circuit. I've managed to pull 6" long arcs from a 15 kV tranny with
just the right (wrong?) amount of capacitive loading - right before the blue
smoke leaks out! I'm allergic to electrons and so I take such measures
seriously.

To use, simply hook the leads in series with the load and clamp the clamp-on
meter through the hole. The meter will read 100X the actual current. Thus 30
mA will read 3 amps on the clamp-on.

There are two barriers to the high voltage involved. One is the epoxy potting
which is an excellent dielectric. The second is the insulation on the
clamp-on meter itself.

For the home-built Ar/Kr ion, HeHg, CuCl/CuBr, and PMG lasers, the excitation
is in the form of short high current pulses. It may also be useful to know
the peak current through the flashlamp of the dye laser (or for other
applications). DC or averaging meters are not very useful for these types of
power supplies. However, if you have an oscilloscope, measurement of both the
peak values and shape of the current pulses can be easily performed:

If the return of your HV power supply is grounded, put a .1 ohm resistor
between the cathode of the laser tube and ground. The voltage across the
resistor will be .1 V/A of tube current.

If the HV supply isn't grounded (e.g., a bridge rectifier), a well
insulated current probe can be used. If you don't have one, a basic current
probe can be built by passing the return lead through a small ferrite core
(either a toroid or even one of those split ferrite RFI suppresssion thingies
found on monitor and other computer cables). Wrap 10 turns as a secondary
on the same core and put it across a 2 ohm load resistor (none of these
values are terribly critical). I would expect the sensitivity to be about
0.2 V/A though this may not be exact. It would be best to check calibration
with a pulse generator. (Note that these sort of current probes - either
commercial or home-built - do not have any response at low frequencies worth
mentioning so they cannot be used for line frequency or DC measurements.)

It is essential that there be some kind of overcurrent protection for any
of the power supplies used with home-built lasers. At a minimum, there should
be a suitably sized fuse or single pole circuit breaker in the Hot side ONLY
of the primary circuit from the AC power line. A time delay fuse or delayed
action breaker may be desirable where capacitive loads are involved and will
minimize nuisance tripping without undo risk to circuitry. A dual pole
ganged breaker can also be used interrupting both the Hot and Neutral.
However, separate fuses should NEVER be installed this way since if only the
one in the Neutral line blows, the entire system will be electrically Hot!

Protecting various subsystems separately is also a good idea since the fuse
or breaker current ratings can more closely match the actual operating values
and thus will be more likely to blow or trip with a fault that the large main
fuse would happily ignore.

Note that it is not necessary to separately fuse the high voltage secondaries
of these systems. For normal transformers, primary protection is adequate.
For neon sign transformers, the output can be shorted all day without harm to
the transformer.

Once can also consider the use of thermal fuses or thermal protectors where
something like a transformer or possibly even the laser tube itself could
overheat.

The purpose of a bleeder is to drain the charge on your lethal power supply
capacitors in a timely manner once primary power is removed. For commercial
equipment, this may mean "by the time some moron can extract 25 screws
and climb inside". For home-built equipment, it will vary from a fraction
of a second to overnight. Of course, a bleeder won't protect you while power
is on!

There are two types of bleeders (these are my terms):

Passive: This is the most common and is normally just a suitable
resistance across each capacitor or the entire filter or pulse forming
network. However, the bleeder resistance wastes power while the system is
energized and must be rated for continuous duty.

Active: Some sort of circuit is provided to connect the resistance
or other load only when power is removed. Most commonly, this would be a
relay whose coil is attached to the switched power with its normally closed
(NC) contacts used to switch in the bleeder. (Note that the relay contacts
need to be rated to hold off the maximum system voltage and to be able to
handle the initial discharge current.) The advantage of this approach
is that little power is wasted while the system is energized and the power
rating of the bleeder resistance can be much lower since it only needs to
absorb the stored energy in the capacitor(s), not the continuous power.

In both cases, the main issue is determining suitable resistance, power,
and voltage ratings for the load.

Resistance: The basic equation for the time constant of an RC
circuit is what determines how fast the capacitor will discharge. This is
T = R * C and is the time (in seconds) it takes a capacitance (in Farads) to
discharge through a resistance (in Ohms) to 1/e (e = 2.71828..., about 37%)
of its initial voltage. The discharge equation is:

Vf = Vi * e-(t/(R * C))

Or, solving for t given final voltage, Vf; or R given Vf and t:

Vi t
t = R * C * ln(----) R = --------------
Vf Vi
C * ln(----)
Vf

The last equation is probably the easiest to use - just determine what the
safe voltage is and how long you want to wait for it!

Note that with respect to shock hazard, 30 V is considered safe but
accidently discharging a cap charged to 30 V suddenly to some other circuit
element may cause damage, especially in systems with solid state components.
Thus, it may be desirable to choose a lower voltage when selecting your
bleeder.

Power: For a passive bleeder, the power in Watts is determined by
P = V2/R. Using a resistance with a power dissipation rating
at least 1.5 times the calculated value is recommended unless forced-air (or
water!) cooling is used. For an active bleeder, the power rating can be much
lower since the resistaors only need to dissipate the energy contained in the
capacitor(s) and this is likely to be a low duty cycle operation. A power
rating lower than 1/10th the continuous rating may be adequate. This will
depend on the type of resistor since they have peak power and total energy
ratings.

Voltage: Yes, resistors also have voltage ratings! For common
carbon or metal film, or even larger wirewound types, this is typically in
the 200 to 500 V range. Thus, for high voltage bleeders, either special high
voltage resistors will be required or the bleeder will need to be made up of
several lower value resistors in series or series/parallel. As a side
benefit, this also makes it easier to achieve high power ratings with low
cost resistors.

In addition to the bleeder, there should be some visual indication of charge
on the capacitors! Bleeders and connections can fail. Always confirm that
the voltage is indeed low enough to be safe BEFORE touching anything!

The design of the neon sign transformer (and Variac) is probably at least 50
years old at this point. Why not replace it with something high-tech and much
more efficient?

After the safety concerns - which are significantly more serious than for
simple transformer based designs - there is a major problem with using SMPSs
to powering experimental apparatus: They like to turn into slag instantly
under conditions which neon sign transformers on Variacs would be perfectly
happy tolerating for hours. :( Unless you spend the time and money to design
in very good protection schemes - not present on something you're likely to
rip out of another piece of equipment where the supply ran under relatively
controlled conditions - you will be doing more repair and rework than playing
with lasers!

SMPSs generally require a minimum load, protection for open circuit and short
circuit/over current faults, they are difficult to design, custom magnetic
components need to be bought or built, they are generally line connected with
large primary side filter caps which makes troubleshooting dangerous, etc.

If you are still thinking about just using a commercial SMPS:

(From: John De Armond (johngd@bellsouth.net).)

Having both tried to repair 'em and modify them for other projects, hacking
commercially produced SMPSs can be done but here are some considerations:

Everything is at line voltage. That makes using grounded instruments
such as scopes a problem. "Use an isolation transformer" is your first
thought but...

Everything is filled with high frequency noise. The TV-type isolation
transformers have enough capacitance to ground and inter-winding capacitance
to make this noise appear on everything. I've had better luck with battery
powered scopes like my Fluke 97 but still the capacitance to ground is
asymmetrical (more on the ground side, naturally) which still turns common
mode noise voltage into real noise.

Modern SMPS's are a study in 'the mostest for the leastest'. Kind of like
working on the last of the tube type TVs where a single tube was pressed into
doing forty things. Components are so interconnected and things interact
and feed back so that diagnostics come down to mostly SWAGing it (Scientific
Wild-Assed Guessing).

The rectified line voltage is sitting there in rather large caps at over
300 volts. It's not like a neon sign transformer or even a small laser power
supply. Crowbar one with a test lead and the tip vaporizes. Crowbar it with
your fingers and you pick yourself up off the floor. Just ask me how I
know. :(

Almost all the parts are house numbered so you'll have few choices in
figuring component ratings. Or even what some of the mysterious epoxy blobs
even do.

That said, SMPSs are the way to go for many things. I'd suggest an alternate
route to hacking a commercial unit. Contact Maxim semiconductors. They're on
the net. For the asking, they'll give you a databook on CDROM which is
excellent. They'll also sample parts with little hassle. Maxim makes a wide
variety of SMPS chips. Their application notes are superb, as are their
ready-to-build designs. You could whip something out on a perfboard faster
than you could get set up to work on commercially made SMPSs.

(From: Sam.)

Or buy one:

(From Jeff Zurkow (jeff@atrox.com).)

In the course of looking for an NST, I discovered the following line of
flyback-type HV power supplies:

For example: the Evertron 2610 is rated at 10 kVAC, 10 mA for $45.50. The
model 2610D has dimming for $56.50. There are also 3.5 kVAC and 6 kVAC
models that are somewhat less expensive. The 3.5 kV unit runs on 12 VDC,
the others on 115 VAC.

The neon bender I visited was kind enough to give me a couple of older
units - one made by Evertron (Everbrite Electronics, the model 3210) and the
other by Transfotek international. One of these (the Evertron) works, but he
had a whole pile of dead ones from various makers. He considers all of the
electronic ones unreliable (compared to conventional NSTs), but that's
probably in 24/7 service. They ought to be OK for intermittent use in
laser and HV projects if the output voltage and current are sufficient.

Evertron Model 3210 Gas Tube Power Supply shows
the schematic of this unit. It has a pair of power MOSFETs driving a
flyback style high voltage transformer, with a whole bunch of open-wound
primaries and a potted secondary. A pair of MOSFETs (Q1, Q2, IRF730s) in a
half bridge configuration drive the primary of a high frequency ferrite
transformer via the coupling capacitor, C3. Oscillation is sustained via a
pair of feedback windings, one for each gate circuit. The pulse width of
the drive to the MOSFETs alternates on the half-cycles of the AC line waveform
via a pair of opto-isolators (U1, U2). Presumably, this creates an effect
similar to the 60 Hz AC of an iron neon sign transformer. Upon power-on, C4
and D6 form a relaxation oscillator with a frequency of about 1 Hz to "tickle"
Q2 into starting. This also activates Q1 via its feedback winding and the
system then comes up (or blows up). :) We (Sam and myself) have are not sure
what the triac circuit does - perhaps it is to detect a broken or shorted tube
condition and inhibit full operation, or perhaps it is there just to confuse
us attempting to reverse engineer the circuit! Or, more likely, it is a sort
of HV GFCI (Ground Fault Circuit Interrupter) - The triac would be triggered
only if there were an unbalanced condition resulting from a short to ground
in (one side of) the HV circuit.

I did plug the thing in and was rewarded with an impressive arc at about 1
cm electrode spacing (bare wires).

The Transfotek unit is completely potted, except for the AC input and
on-off switch. And completely dead.

It may be possible to feed the output of one of these units to a HV multiplier
(home-built or one from a TV) to boost it to 20 or 30 kV. The only problem
with using a commercial TV tripler would be that their design current is a
couple of mA - I don't know how well it would hold up with much higher output
current.

(From: Sam.)

The only problem with using a a TV tripler may be its rated current is only
a couple of mA and I don't know how well a typical unit it would hold up with
much higher output current. Attempting to power a higher current laser may
result in mediocre performance or early failure.

A triggered spark gap may be desirable as a means of providing a very short
high current pulse for some versions of the N2, CuCl/CuBr, PMG, and other
pulsed lasers. The basic concept is similar to the triggering of a xenon
strobe expect that the desired effect is the current pulse rather than light.
In fact, a xenon flashlamp can be used as the trigger device in some types of
triggered spark gaps,

(From: Jonathon Caywood (sarlock@twcny.rr.com).)

So far I have turned up several triggered spark gap (SG) designs as listed
below.

Close proximity smaller SG next to the larger SG. This design employs
constructing an secondary electronic driven SG which sits next to the main
SG. From what I gather this design uses the UV from the secondary SG to
ionize the air between the electrodes on the primary SG, causing it to
trigger.

Three electrode configuration. This basically consists of your two main
SG terminals with a third 'initiator' terminal placed in between the main
SG terminals. In some designs the initiator terminal is offset and closer
to one of the main SG terminals, in other designs the initiator terminal
sits directly in the middle of the two SG terminals

Laser triggered SG. I included this one for fun of it since it's not
really useful for my particular application, but is interesting. Basically
a nitrogen laser is used to ionize the air between the SG electrodes and
cause it to fire. This only seems useful for larger spark gaps where a
giant thyratron is out of the question, since you need a triggered spark gap
to get the proper timing on the firing of the nitrogen laser.

(From: Mark Kinsler (kinsler@frognet.net).)

I horsed with these in my high-voltage days and found that there's a bit
more art than science in getting them to work. I don't think that the UV
is that big an issue, though there's still some debate about it. Dr.
Michael Mazzola of Mississippi State University did some research on the
role of UV in spark gap triggering fairly recently.

I think that methods 1 and 2 are essentially the same, actually. You can
also think of the xenon photoflash circuit in a camera as being about the
same thing: there's a little wire wrapped around the strobe tube about
halfway between the main terminals. This wire is supplied with a 4kV
pulse from the shutter circuit.

The triggered spark gap on our old Marx generator at MSU consisted of two
toilet-tank-sorts of copper spheres. A 1" or so hole was cut into one of
the spheres. In the middle of this hole you could see a long needle
electrode: it looked sort of like the sphere had a tongue. This needle
was contained inside but insulated from the the sphere. A high-voltage
pulse (20kV?) was used to make a spark from the needle to the sphere that
surrounded it. This would initiate the discharge between the two spheres
quite reliably.